Re: Bridging the digital voice and data gap
"John D. Hays" <john@...>
Hi Tyrell,toggle quoted messageShow quoted text
Vocoders and other codecs are different beasts.
Two short articles:
In layman's terms, a codec converts an analog stream into a coding format. Some coding formats are lossless and produce a true representation of the orignal analog stream in a digital format, but most such as MP4 and AAC, use compression to get a lower data rate.
A vocoder is considered a type of codec, in that it is converting "analog" voice to what is usually a very low data rate bitstream and back. However, it's methodology is fundamentally different than traditional audio lossy and lossless codecs. AMBE and CODEC2 both approach the problem by detecting and recognizing phonemes (the basic sounds that make up human speech) and such things as frequency slope, amplitude, and inflection in the analog stream. The vocoder then represents the phonemes and modifiers as digital codes (e.g. 0x3E might represent the 'th' sound), which are transmitted digitally, where the receiving end uses those codes to synthesize the original speech. This process is focused on human speech and has the side effect of mostly ignoring sounds that are not human speech.
The semantic is important to differentiate both the technique and result of the method of encoding and decoding speech. If you transcoded an MP3 (lossy) music stream into AMBE or Codec2 the result would likely be unrecognizable.
It may be that transcoding between two similar vocoders would exhibit less deterioration of a voice signal, since they would both be looking for the same attributes in the "audio" signal. We won't really know until someone tries it in the real world.
Everything else on the "data" side of a complex DV radio protocol can probably be handled, in one form or another, though there would be mismatches. For example, going from D-STAR to DMR, "callsign addresses" and unit identifiers could be mapped to one another. On the other hand, there is no equivalent to D-STAR's ancillary data (e.g. comment text, D-PRS, etc.) in IRLP's network, though a SIP link could send it as a message in the control channel.
On Fri, Aug 3, 2012 at 7:00 AM, Tyrell Berry <kd7kuj@...> wrote: