Slashdot Mirror
Open Source Codec Encodes Voice Into Only 700 Bits Per Second (rowetel.com)
Longtime Slashdot reader Bruce Perens writes: David Rowe VK5DGR has been working on ultra-low-bandwidth digital voice codecs for years, and his latest quest has been to come up with a digital codec that would compete well with single-sideband modulation used by ham contesters to score the longest-distance communications using HF radio. A new codec records clear, but not hi-fi, voice in 700 bits per second -- that's 88 bytes per second. Connected to an already-existing Open Source digital modem, it might beat SSB. Obviously there are other uses for recording voice at ultra-low-bandwidth. Many smartphones could record your voice for your entire life using their existing storage. A single IP packet could carry 15 seconds of speech. Ultra-low-bandwidth codecs don't help conventional VoIP, though. The payload size for low-latency voice is only a few bytes, and the packet overhead will be at least 10 times that size.
I wonder how it performs on tonal languages like Cantonese.
http://michaelsmith.id.au
That's starting to approach feeding the sentence into a speech to text system at one end and then sending the text over the air to be fed back into a text to speed converter.
I read the internet for the articles.
Encoding voice more efficiently has implications far exceeding the amount of storage space required to save it. There's a reason why the article is comparing the new codec to single sideband. When transmitting digital data over radio, it pretty much invariably (nowadays) means some sort of spread spectrum transmission. The fewer bits required per second means the less spectrum you are having to spread your signal over, this the more concentrated your signal is. A radio transmitter has a fixed power output, so if you are smearing that power over less band, then you have a stronger signal.
It is a testament to the amateur radio pioneers of the past that an analog radio transmission mode invented over a hundred years ago is, just now, being possibly rivaled in its efficiency.
To compute the channel capacity, you need to know the channel's signal-to-noise ratio as well as its bandwidth.
The Shannon channel capacity formula is: C = B * log_2(1 + SNR) where C is the channel's capacity in bits/second, B is its bandwidth in hertz, log_2 is the base-2 logarithm and SNR is the channel's signal-to-noise ratio.
If we assume an SNR of 48 dB for a reasonable POTS line, its capacity would be C = 3 kHz * log_2(1 + 48 dB) ~= 3000 * log_2(63097) which is almost 48,000 bits per second.
This is a theoretical limit that realizable systems can only approach, but never equal or exceed. A practical system would also use extra bits for forward error correction purposes; I doubt that this codec deals gracefully with bit errors.
For back-of-the-envelope purposes, assume you could use this codec to send a single voice signal in 700 Hz of bandwidth on a channel with low SNR, or you could send 60 voice signals over a regular POTS line.
Actually, our modems degrade gracefully. The least-protected bits go wrong with low bit-error rates, and the more protected bits survive. It takes a high bit error rate to kill it. So bit errors result in the speech being "off" but not dropping out.
Bruce Perens.
In other words, it's being efficient.
The brain has a very powerful voice and audio decoder. (In fact, the brain's wetware is so powerful to compensate for relatively poor sensors - but coupled with the power of the brain, they become much more powerful detection devices. The downside to the economy in hardware with powerful software combination is artifacting - though we usually call those things illusions).
So the codec basically saves transmission bytes by making the brain do a lot of the signal recovery work.
Of course, in Amateur Radio, SSB can be really bad and you have to do a lot of deciphering anyhow.
That's who'll be interested in technology like this. They could compress and store the conversations of every person in the U.S., 24/7/365, for decades, without having to upgrade their data storage capacity.
Just to show I'm not all gloom-and-doom: I'd think NASA, and private spaceflight companies like SpaceX, would be interested, since a low datarate for voice communications would be great, I'd think, for interplanetary distances. With higher datarates available you could have multiple conversations happening simultaneously.