Voice at 700 Bits Per Second

All other things being equal, signals with wider bandwidth can carry more information. Sometimes that information is data, but sometimes it is frequency. AM radio stations (traditionally) used about 30 kHz of bandwidth, while FM stations consume nearly 200 kHz. Analog video signals used to take up even more space. However, your brain is a great signal processor. To understand speech, you don’t need very high fidelity reproduction.

Radio operators have made use of that fact for years. Traditional shortwave broadcasts eat up about 10kHz of bandwidth, but by stripping off the carrier and one sideband, you can squeeze the voice into about 3 kHz and it still is intelligible. Typical voice codecs (that is, something that converts speech to digital data and back) use anywhere from about 6 kbps to 64 kbps.

[David Rowe] wants to change that. He’s working on a codec for ham radio use that can compress voice to 700 bits per second. He is trying to keep the sound quality similar to his existing 1,300 bit per second codec and you can hear sound samples from both in his post. You’ll notice the voices sound almost like old-fashioned speech synthesis, but it is intelligible.

Your ears are not linear with respect to frequency response, and the codec takes advantage of this, sampling more low frequencies than high frequencies. There are other specialized signal processing and filtering steps taken to improve the audio quality. Here’s the block diagram (you can find out more at the original post):


Tight bandwidth has lots of advantages: more channels per given frequency band, less interference, and improved resistance to noise. A 700 Hz audio signal able to carry speech would have major implications for radio communications by voice.

The codec is destined for integration with FreeDV. We’ve talked before about the wealth of technology ham radio produces. Perhaps others will find use for this codec in other situations that are not ham radio-related.

Thanks to [Rob] for pointing this out.

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