Suppose two people want to transmit some binary data (a png image perhaps) by voice.

Two of my characters want to share binary data but they can only use their voice and no other form of communication is possible.

By "voice", I mean any sounds that can be reliably produced and differentiated by average humans.

They can use computers to encode or decode the message but the transfer of information needs to happen between them. For example: They can't use computers to encode the data to sound, play it back and let a computer on the other side record it.

They could do it by pronouncing every single zero and one. ("One, Zero, Zero, One, ..") This is very slow and inefficient though.

Or they could use the hex encoding. ("B, Four, F, Nine, ...") This is better, but there is still more room for improvement.

Perhaps they could use base64 to encode it. ("D, capital G, H, three, ...") But notice how they have to specify capital letters ("capital D"), this lessens it efficiency.

What's the most efficient way to do it?

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Commented Apr 15, 2021 at 15:46
  • $\begingroup$ Can the receiver record the sounds for interpretation by a computer, or do they need to write it down? $\endgroup$
    – wizzwizz4
    Commented Apr 16, 2021 at 13:12
  • $\begingroup$ @wizzwizz4 From my question: They can't use computers to encode the data to sound, play it back and let a computer on the other side record it. So no, they can't record it and must write it down $\endgroup$
    – Cookie04
    Commented Apr 16, 2021 at 14:58
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    $\begingroup$ @Cookie04 Thanks for the clarification. In my dialect of English that would be “or”; I thought you were referring to the entire process being disallowed. $\endgroup$
    – wizzwizz4
    Commented Apr 16, 2021 at 15:37
  • $\begingroup$ Note: A PNG image is massive when transformed into binary-voice data. Even a stupidly simple image, like say - a 20 x 20 black and white picture - can end up a couple hundred bytes in length and be very hard to memorize or speak. If you plan to send conspiracy theories, gold maps or illegal porn over this method, it will take quite a while... $\endgroup$
    – Mermaker
    Commented Apr 16, 2021 at 19:14

11 Answers 11


This problem has been much studied. While it may be difficult to define what 'best' is, there is a very efficient one already in existence, and that's PGP Words. This method was designed for transmitting long binary keys over a voice link, each word encoding a whole 8 bit byte.

It addresses a number of problems you probably haven't thought of, like reliability over a voice link. What happens if a word is missed, or a repetition for clarity is mistaken as an actual repetition or, reading from a long list, two words get swapped?

With 8 bits per word, you would normally require 256 words. The system is more sophisticated than that, and uses 512 words, an 'even' table of two syllable words, and an 'odd' table with three syllables, that are used alternately. That way, a missed or repeated word, or two swapped words, can be immediately spotted as an error.

Here are the first few and last few, from the wikipedia article linked above. The whole table can be printed on a single sheet of A4. They are in alphabetical order to aid the receiver. Obviously the list is optimised for English. Speakers of other languages may prefer a different list.

Hex  Even Word  Odd Word
---  ---------  --------
00   aardvark   adroitness
01   absurd     adviser
02   accrue     aftermath
03   acme       aggregate
04   adrift     alkali 

..  ......      .......

FB   watchword  Wichita
FC   wayside    Wilmington
FD   willow     Wyoming
FE   woodlark   yesteryear
FF   Zulu       Yucatan 

Quote from the wikipedia article

The PGP Word List was designed in 1995 by Patrick Juola, a computational linguist, and Philip Zimmermann, creator of PGP. The words were carefully chosen for their phonetic distinctiveness, using genetic algorithms to select lists of words that had optimum separations in phoneme space. The candidate word lists were randomly drawn from Grady Ward's Moby Pronunciator list as raw material for the search, successively refined by the genetic algorithms. The automated search converged to an optimized solution in about 40 hours on a DEC Alpha, a particularly fast machine in that era.

An alternative for a 4 bit nybble per word is simply to use the hex alphabet, perhaps using the ICAO/NATO pronunciation, 'zero' to 'niner' then 'alpha' through to 'foxtrot'. There are more than 16 further letters left if needed for even/odd coding. Whether the complexity of the 512 words needed for doubling the throughput with PGP words is warranted against the simplicity of hex begs the question of how you define 'best', what factors in the setup or operation of the communication are important.

You could get even higher efficiency by using more bits per word. 12 bits would need a 4096/8192 long dictionary. This would sacrifice much of the hard-won inter-word phonetic distance of the PGP scheme, so would require a higher fidelity voice channel, and more careful speakers.

Noting ruakh's comment, it's worth looking at the speed of the channel. His estimate is two seconds per word, which would probably be quite good for untrained users. That's 4 bits/s. If we compare that with Morse code, the minimum speed required by the FCC to grant a radio operator's license used to be 16 five-letter code groups per minute, which very roughly equates to about 8 bits/s.

The difference between the two systems is that Morse Code requires training. I couldn't transcribe Morse, at any speed, without a lot of practice, and probably some tuition as well. Many English speakers could transcribe those words without practice, but what about those with a limited vocabulary, or English as a second language, or speakers of other languages? PGP words is not really training-free, if it's to be used by any human at all. It's only ready-to-go if used by people like those who invented it, educated fluent English speakers, being a programmer would help as well. It's probably a skill that's easier to pick up than Morse though.

With speed in mind, it might be worth reviewing the performance of hexadecimal via ICAO pronunciation. While a word every two seconds would be good going for recording a PGP word manually, I think hexadecimal could be done at easily twice that rate, transcribing as you go, making the bit rate of the two methods equivalent.

Clearly a lot of other assumptions about the training or experience of users, the setup costs, the quality of the audio link, have to be defined before the best system can be determined.

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    $\begingroup$ Nice answer, learned something new today $\endgroup$
    – MolbOrg
    Commented Apr 14, 2021 at 15:39
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    $\begingroup$ Awesome! Always cool to see a worldbuilding problem that it turns out people have already been working on. $\endgroup$
    – Qami
    Commented Apr 14, 2021 at 15:45
  • $\begingroup$ +1. It may be worth pointing out that although this is much more efficient than reciting a sequence of "zero" and "one", it is still quite slow -- at a rough handwavy guess, maybe half an hour per kilobyte? (That's assuming about two seconds for the speaker to determine the word to say and then say it, or for the listener to identify the word that was said and then transcribe either the word or the byte.) I hope the characters in this story are getting something really nice in exchange for spending so much time doing something so boring! $\endgroup$
    – ruakh
    Commented Apr 15, 2021 at 2:56
  • $\begingroup$ i bet you could transcribe binary morse! dit=0 dah=1 (or vice versa) $\endgroup$ Commented Apr 16, 2021 at 14:00
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    $\begingroup$ @user253751: You could, but that's 1 bit per syllable; very inefficient. PGP Words is 3.2 bits per syllable. It should be theoretically easy to get up to 7 bits per syllable, if we lose the error correction. $\endgroup$ Commented Apr 16, 2021 at 14:57

Efficiency means something that is easy/quick to encode, easy/quick to pronounce and understand, and easy/quick to decode. base64 was never made to be read by a human, and saying "capital" every now and then will seriously slow you down. Hexadecimal is slightly better than spelling binaries, but perhaps too short. Seven are two syllables.

  • Define 32 (or 64 or 128) short, distinctive words. Which ones depend on the languages and accents of the participants. Cat, Dog, Fox, ... but if Cat is in the list then Bat should not be there, or vice versa.
    The suggestion by the commenter Zeiss Icon to use NPA has merit, but it is limited to 5-bit units. Finding 64 words might be feasible ...
  • Assign each word a number.
  • Break the binary data into "bytes" of 5 (or 6 or 7) bits, encode and read the words.
  • Listen to the words and then decode into "bytes."

It might be a good idea (even if it decreases efficiency) to add a checksum to your transmission.

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    $\begingroup$ There is an existing 36-character set of very distinct symbols: the Internation Phonetic Alphabet. Alpha, Bravo, Charlie, Delta, Echo, etc. along with the numbers Zero through Niner. This gives five bits for each character, plus leaving a few for special characters. This set was created specifically to avoid confusion of one character for another (hence by Bravo, Charlie, Delta, and Tango sound very different, for instance). Further, they're distinguishable even with an accent, a fair amount of noise, and to those who aren't native English speakers. $\endgroup$
    – Zeiss Ikon
    Commented Apr 13, 2021 at 18:28
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    $\begingroup$ @ZeissIkon Ok, this needs to stop. The acceptable alternative names for the International Radiotelephony Spelling Alphabet (the Alpha, Bravo, Charlie... thing) are the NATO phonetic alphabet, the NATO spelling alphabet, the ICAO phonetic alphabet and the ICAO spelling alphabet. Note that International Phonetic Alphabet (IPA for short) is not on the list. That's because the IPA is something completely different that is not at all suited for this. $\endgroup$
    – No Name
    Commented Apr 14, 2021 at 11:07
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    $\begingroup$ @NoName Not going to disagree, but as a US licensed amateur radio operator ("ham") I've never heard the terminology you give. Perhaps, like many other things, the "wrong" way is far more common than the "right" -- but if you use one of those other terms, few if any who actually use this thing will know what you're talking about out of context. $\endgroup$
    – Zeiss Ikon
    Commented Apr 14, 2021 at 12:39
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    $\begingroup$ @ZeissIkon I know what the NATO Alphabet is, yes - I'm a navy brat. All I'm saying is that the IPA I, and a good fraction of the people around this particular stack, know is a system for transcribing speech into writing in a way that indicates the sounds spoken without need for an audio recording, rather than a method of transmitting alphanumeric serial numbers over a noisy media channel without loss of information. Wiki link for your reading pleasure: en.wikipedia.org/wiki/International_Phonetic_Alphabet $\endgroup$
    – No Name
    Commented Apr 14, 2021 at 15:21
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    $\begingroup$ @NoName Oh, that thing. Yep, completely useless for this need. The ICAO one is what's needed here. $\endgroup$
    – Zeiss Ikon
    Commented Apr 14, 2021 at 15:51

Choose X syllables, and then base-X-encode your data.

Choose the largest set of syllables that you consider for your purposes to be mutually distinguishable, and then base-X-encode your binary data, where X is the number of syllables you've chosen.

A simple example: suppose we choose only syllables that begin with a consonant, and are followed by a vowel, from the following sets:



We now have 8 x 5 combinations, giving us 40 syllables. You can base-40 encode your binary data into a rapid-fire of syllables that would sound something like:

"pagidachotajapikachutagujiko.... "

This example used latin-alphabet letters and assumed a more-or-less standard English pronunciation. To make a more rigorous system, it would be advisable to use the International Phonetic Alphabet to choose the sounds for your syllables and ensure that you avoid phonemes that (1) would be difficult to distinguish from one another and (2) do not lend themselves to fast pronunciation.

Note: This answer is similar to o.m.'s...just "encoding" at the level of syllables instead of words. By customizing your list of syllables to all be quickly-speakable and using every possible combination, I expect this would tend to increase the efficiency of information per syllable. However, if the memory and cognitive processing rate of the average humans using it is taken into account, it's possible (probable?) that o.m.'s word-sequence-based transmissions will be more easily remembered without error.

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    $\begingroup$ beep boop, trademark violation detected! please pay $1 royalty to Nintendo Corporation for use of the word "pikachu" in your phone call. This has been a message from galactic cyber-enforcers inc. $\endgroup$ Commented Apr 14, 2021 at 11:11
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    $\begingroup$ @user253751 Deliberate easter egg - well spotted! I'll pay the $1. ;) $\endgroup$
    – Qami
    Commented Apr 14, 2021 at 13:21
  • $\begingroup$ btw "j" and "ch" are too similar, so don't use both $\endgroup$ Commented Apr 14, 2021 at 13:48
  • $\begingroup$ @user253751 Well, the same could be argued about the other pairs (the last four consonants are just the voiced versions of the first four). But it's up to the OP to decide, based on intended usage, what's distinguishable... $\endgroup$
    – Qami
    Commented Apr 14, 2021 at 15:07
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    $\begingroup$ This approach does sound an awful lot like Bubble Babble, referenced in this stackoverflow question, defined in this IETF draft, implemented in OpenSSH for fingerprint validation over voice channels. $\endgroup$
    – orithena
    Commented Apr 15, 2021 at 13:34

Sing it

You're thinking too much along programming lines. This is valuable for the sake of compression: but everything else you've mentioned (like hex encoding or base64) is just a way to make binary more consumable by the human eye (and other things).

Frankly, from a programmatic perspective, the only thing you really care about is compression. You want to send as little data as possible to guarantee maximum transmissibility. But other than compression, it doesn't matter how you express your ones and zeros. (It helps to remember the good old days on the Apple II computers where the average geek cared about Assembly Language.)

What you really want to think about is music

If you want the average human to convey ones and zeros, ask them to sing. Generally speaking there are only seven notes — but there are half notes, quarter notes, sharps and flats. Those alone give you 63 notes. Add shifts in octave and you get more. Your average person can span two octaves. Now we're up to 126 notes. 127 with a "pause" (no sung note in the meter of the song). Expand the vocal range just a hair and you get to all 128 positions, allowing you to express every combination of "11111111" or 27.

BTW, I'm not a music expert. I wouldn't be at all surprised that you could express a whole lot more data than 27 with the magnificent expressiveness of musical notes.

From here, it's just a question of mapping notes to binary combinations and, boom, vocally expressed digital data.

Finally, I'm not suggesting that what you get would sound good... only that it could be done.

enter image description here

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    $\begingroup$ Interesting idea. Wouldn't it be difficult for a non-musician to hit those notes and correctly identify each note though? If someone would sing a note, I probably couldn't tell you which one. As I said in my question I need a solution that works for the average human. Not sure if the average human could do that $\endgroup$
    – Cookie04
    Commented Apr 13, 2021 at 19:34
  • $\begingroup$ @Cookie04 Your average person can follow simple to moderately complex songs fairly well. Add lyrics (using an averaging algorithm to "hit the note') and it's even easier. The distinction between each note on the vocal scale is quite high, so we're back to an averaging algorithm (already solved, there are apps that make your singing sound better, that's the basis for the tech to "decode" the data). Frankly, I can't see how this wouldn't work with one possible exception: some note shifts would require a slower tempo or the human voice can't make the shift. But, just slow it down. $\endgroup$
    – JBH
    Commented Apr 13, 2021 at 19:38
  • $\begingroup$ I'm somewhat musically talented, and I wouldn't be able to transcribe this very fast. You don't need (and don't want, due to differing vocal abilities) to use absolute pitches, so having perfect pitch is not required, but even so, some people may not be able to work out relative pitches, especially if you're throwing sharps into the mix. (Note that there is no such thing as sharps and flats. Yes, pedantically, I'm simplifying a bit, but in practice you can't expect to tell an a-sharp from a b-flat.) $\endgroup$
    – Matthew
    Commented Apr 13, 2021 at 19:45
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    $\begingroup$ @Matthew, you're absolutely right... you're also taking all the fun out of this. Nobody in their right mind would convey digital data by voice. Not under any circumstances. But since we're dealing with a fictional world with fictional rules... I think this would be fun. Could you imagine a society that had become reasonably adept at this - and then we defenseless humans met them? Shaka, when the walls fell... $\endgroup$
    – JBH
    Commented Apr 14, 2021 at 2:18
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    $\begingroup$ This is musically and mathematically quite an overestimate - There are 12 semitones in an octave, including sharps and flats (which are the half-notes), so two octaves gives you about 24 notes, or 48 if you allow quarter-notes (which I think most people would not be able to reliably sing or discern), not 63. Using a pause is also problematic if the data happens to encode to multiple pauses in a row, it may not be easy for the listener to decode exactly how long the pause was. Even so, 128 is 2^7, not 2^8. $\endgroup$
    – kaya3
    Commented Apr 14, 2021 at 8:11

The best way to communicate binary information long distance by human "vocal" is by a whistle language. The existing whistle languages are used in rough landscapes to communicate longer distances than possible through voice.

In this case, you can have one pitch for one and another for zero (or work up a common set of tones for numbers 0 through 7).

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    $\begingroup$ Nice to see that here, even if it not necessarly fits op's situation $\endgroup$
    – MolbOrg
    Commented Apr 14, 2021 at 15:29

Human speech is astonishingly effective as establishing communication between humans. Not only does it use the vocal chords efficiently, but it uses the language centers of the brain efficiently as well. Our languages have just enough redundancy to support humans.

Accordingly, the best way to have an average human transmit and recieve binary data is to map it to human language. Use an AI to construct an injective mapping between a string of bits and sentences in English (or whatever the native language of your characters is). The result should sound like a typical monologue.

This is almost certainly the most efficient way to communicate a binary string between people. If one side has other tools available , such as a tape recorder, there may be more efficient means But never underestimate the benefits of leveraging a few decades of speech on behalf of both parties.

  • $\begingroup$ Amusingly enough, the resulting encoded binary should look something like the content of the answer itself... although probably more nonsensical (or at least, I hope what I wrote is more sensical than the encoded content would be!) $\endgroup$
    – Cort Ammon
    Commented Apr 14, 2021 at 13:29
  • $\begingroup$ Yeah, markow chans back in business, woohoo... Was looking for the answer $\endgroup$
    – MolbOrg
    Commented Apr 14, 2021 at 15:34

What you need is a drum. Actually, any old stick and a hard surface to strike it on will do in a pinch, but that limits you to binary data ("hit" or "no hit") and doesn't allow for any out-of-band information such as "end of message". A drum that makes a different sound on sustains (the stick is kept in contact) vs. rests (the drum is allowed to vibrate on its own) will let you have notes of varying lengths which can form more complex codes. Drumbeats can be very fast and accurate, far more than most singers, and can send longer messages without pausing.

For instance, with such a drum, you could have a code of "short" and "long" beats that encodes letters and punctuation; you may know this as Morse code. Or, you could cut to the chase and encode a binary or quaternary (or any other convenient base) number directly as a series of beats.

The advantage and goal here is to keep the encoding and decoding simple, because the limitation is not in the ability of the human body to make noise, but the speaker's ability to figure out which noises they ought to be making. A base-32 or -64 code is more efficient in terms of symbols, but any speed benefit is undercut by the user's own speed in interpreting those symbols as actual sounds.

In contrast, using only three symbols (short, long, and silence), Morse code is a proven example of encoding and decoding a message, by human operator, in real time. So it makes sense to investigate the area of low-density but high-speed communications to make most efficient use of the human part of the equation.

  • $\begingroup$ I specified that I need a system which uses the human voice (Any sound that can be reliably produced and differentiated by the average human) $\endgroup$
    – Cookie04
    Commented Apr 13, 2021 at 18:54
  • $\begingroup$ @Cookie04 Ah, I misunderstood. By "reliably produced" I assumed you just meant it couldn't require any specialized or complex equipment, rather than no equipment at all. $\endgroup$
    – Cadence
    Commented Apr 13, 2021 at 18:58
  • $\begingroup$ Clapping or clicking (e.g. with the tongue) would work in place of a drum. $\endgroup$ Commented Apr 14, 2021 at 4:51
  • $\begingroup$ The main trouble here is that in order to identify a non-sound, you need a reliable clock. $\endgroup$ Commented Apr 14, 2021 at 4:52

I'd encode the binary to a huge lookup table of values and concepts, link these concept and values to phonemes that are easy for the human vocal apparatus to process, and for the human ear to hear.

Group these sounds into more complex structures, for improved bit density.

Ideally, I'd assign some contextual meaning to these complex structures of phonemes, to ensure greater ease of comprehension, and cut down on the error rate when producing them. There would be intricate rules regarding juxtaposition of these structures, disallowing obvious errors. We might need to make lookup tables of these structures, as an aide to learning and for error-checking.

Of course, the art of learning such a highly complex and convoluted data pattern will need to commence from childhood, and continue throughout adult life.

For convenience, we will call this convoluted binary-encoded-as-phonetic-sounds-in-structured-groups-with-contextual-meanings an easier name to identify it.

How about "English"?


I suggest combining the best of o.m.'s accepted answer and JBH's suggestion of singing, to use a spoken tonal approach. Many people use languages in daily life that rely on tone for meaning, so your "average person" may well be able to.

Tone isn't usually employed in English, but combining even a simple high-low distinction, which pretty much anyone can do, with the NATO alphabet's 36 characters gets you 72 distinct values (or 6 bits if you want simpler mapping). I reckon this is optimal for a pair of English speakers with no training.

Three tones (high-mid-low) gives you 108 values (more than enough for something based on ASCII85), but if you can use rising and falling tones too, you get 36×5=180 values. Some Chinese dialects use even more. Unless mapping bits to vocalisations has to be done by people using a look-up table, you don't need your unique vocalisations to add up to a power of two, as demonstrated by ASCII85. The more tones used, the more training will be required, at least if your speakers aren't used to it.

Cooking up a new phonetic alphabet using only one- and two-syllable words is probably helpful for efficiency. Of course you could discard the mapping to the alphabet, but that mapping does makes transcription easier (tone markers will be needed).

You should consider the fidelity of the channel - a quiet room, across a deep gorge with a noisy river, a telephone line, a party, etc. will have different characteristics. Some will need a wider distinction between sounds than others. Only in the quietest settings could volume be used as another variable.

Running some numbers on the rate using simple mappings (i.e. rounding the number of unique sounds down to a power of two): English speech is apparently around 4 syllables per second. Using simple mappings, with 6 bits over two tonal variants of the two-syllable A-Z0-9 alphabet, you'd get 12 b/s. With 5 tones rounded down to 7 bits, but upping the rate to a still-reasonable 6 syllables/second you're now at 21 b/s. Pushing it further, if you can come up with 64 distinct single-syllable words and can manage to apply 4 tones to those sounds you'd get 256 values per syllable, or a grand total of 32b/s. Don't forget to breathe.


There are approximately 470k English words. $\log(470000)/\log(2)$ yields 19 bits per word. Good luck teaching kids all English words and their binary counterparts as well.

At that point, it might be easier to create 256 "words" at 1 byte per word. Words that are syllabically short and enunciated carefully. You could likely do 1-syllable each, so, "cabagathadedodunit", where each syllable is a 1-byte value "word". That string would yield 8 bytes or 64 bits.

If you think your kids are just really damn smart, you could bump that up to 65536 words stowing 2 bytes per word with more syllables per word, but that presents a much greater error risk.


Morse code may be voiced or whistled. Computers can turn the binary data into letters (base 26, or perhaps a lower base by dropping the longest morse codes.)

Those who train, gets their morse up to 60 wpm or so. The average word is about 5 letters. So 300 letters per minute, each encoding 4.7 bits. 1410 bits per minute, or 23.5 bps. Well, that is for morse using equipment. I found no data for voiced morse.

  • $\begingroup$ Voiced Morse is considerably slower. You can't move your mouth fast enough to get more than about two bits per second. $\endgroup$
    – Mark
    Commented Apr 16, 2021 at 22:08
  • $\begingroup$ @Mark have you never heard anything like this youtube.com/watch?v=lINneylEo0U I recon he'd be competitive with PGP words as a bit rate $\endgroup$ Commented Apr 16, 2021 at 22:53

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