Monday, September 5, 2011

D-star, P25, other AMBE/IMBE modes

For some reason people are going around saying that "the DSP chip in D-Star, etc radios is really no different than having a DSP in modern HF radios. People don't suddenly refuse the operate their HF radio because they can't see what the DSP is doing."

This statement is a prime example of wrong thinking. What's worse is the same people explain to me that the only difference is the HF radios use the DSP for "IF-level modulation and detection" while the chips in D-Star are just vocoders.

Okay, what is a vocoder? (I am playing stupid here for the benefit for people who really don't know) defines it as:
vocoder (play /ˈvkdər/, short for voice encoder) is an analysis/synthesis system, mostly used for speech. In the encoder, the input is passed through a multiband filter, each band is passed through an envelope follower, and the control signals from the envelope followers are communicated to the decoder. The decoder applies these (amplitude) control signals to corresponding filters in the (re)synthesizer.
I don't understand the math behind it. Basically, instead of digitizing the speech and sending it directly over.. the vocoder system renders it into a series of much smaller operations that can be done on the other side to recreate the speech.

This quote from backs this up:
vocoder works by analyzing speech and encoding it as phonemes and other higher-level information, rather than as a continuous waveform. It can thus compress speech into substantially less bandwidth than a waveform codec. Vocoders are generally less intelligible than codecs. They reproduce an evocation of the orginal voice, rather than the actual voice, but sophisticated vocoders like AMBE do so well enough that - except at the lowest bandwidths - the speaker's identity is immediately and easily recognizable. 
The problem with that is certain operations can be patented which will make your vocoder unable to be reproduced legally by anyone other than yourself.

Dave Rowe writes on his website:
Proprietary codecs typically have small, novel parts of the algorithm protected by patents. However proprietary codecs also rely heavily on large bodies of public domain work. The patents cover perhaps 5% of the codec algorithms. Proprietary codec designers did not invent most of the algorithms they use in their codec. Typically, the patents just cover enough to make designing an interoperable codec very difficult. These also tend to be the parts that make their codecs sound good.
So, no one can make a codec that will be compatible with DVSI's codecs, but it is possible to make a codec that works just as good. Codec2 is such a project. It sounds pretty good to me right now, but it is just alpha software at the moment.

The only company that can make chips that are compatible with the vocoder in D-Star, P25 and other radios that use AMBE or IMBE is DVSI. It's true that DVSI just provides the programming on the chips and they are DSP chips from TI. You might even find the same model DSP chip in your HF radio. The problem is to communicate by D-Star you have to have a chip from DVSI in both sides of the conversation.

On the other hand, anyone with a homebrew radio that can do modulation in CW/SSB/AM or FM can communicate with any Gee-Whiz HF radio with a DSP inside of it just fine.

Anyone who says that the DSP in a D-Star radio and a DSP in an HF radio is equivalent is acting in a dishonest manner.

As soon as someone makes a legal source code available that I can load onto a fresh DSP from TI or any other DSP company, build into a radio and communicate with D-Star radios, then I'll stop complaining about the chip inside of them. While DVSI is the only source in the world for said chips, then I'm not going to shut up about it.

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