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Hi all

I am implementing an ADPCM codec for use in wireless transmissions (as
frequent blog visitors may already know :). I have a question regarding
channel coding for the wireless channel.

All the codes I know have error detection and (some) error correction
capabilities. So far so good. But for audio transmission I don't really
need to get the exact transmitted values. I don't care if the LSB falls
from time to time. I do care if a MSB or a sign falls. This leads to my
question:

Are there channel codes that provide something like a soft decision on the
transmitted value? Say I transmit an 8 bit value - I transmit the value
113. I need a channel code that tells me "there were errors, but the value
is probably larger than 100 and smaller than 130, so 115 would be a good
guess"

Do you know of any way to implement something like this or are there codes
doing this?

Markus

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dudelmann <d...@gmx.de> wrote:

>I am implementing an ADPCM codec for use in wireless transmissions (as
>frequent blog visitors may already know :). I have a question regarding
>channel coding for the wireless channel.

>All the codes I know have error detection and (some) error correction
>capabilities. So far so good. But for audio transmission I don't really
>need to get the exact transmitted values. I don't care if the LSB falls
>from time to time. I do care if a MSB or a sign falls. This leads to my
>question:

>Are there channel codes that provide something like a soft decision on the
>transmitted value? Say I transmit an 8 bit value - I transmit the value
>113. I need a channel code that tells me "there were errors, but the value
>is probably larger than 100 and smaller than 130, so 115 would be a good
>guess"

Look into UEP (unequal errro protection.  

Steve

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>Look into UEP (unequal errro protection.  
>
>Steve

Aha - didn't know that one. Thx a bunch.

Markus

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dudelmann wrote:
> Hi all
> 
> I am implementing an ADPCM codec for use in wireless transmissions (as
> frequent blog visitors may already know :). I have a question regarding
> channel coding for the wireless channel.
> 
> All the codes I know have error detection and (some) error correction
> capabilities. So far so good. But for audio transmission I don't really
> need to get the exact transmitted values. I don't care if the LSB falls
> from time to time. I do care if a MSB or a sign falls. This leads to my
> question:
> 
> Are there channel codes that provide something like a soft decision on the
> transmitted value? Say I transmit an 8 bit value - I transmit the value
> 113. I need a channel code that tells me "there were errors, but the value
> is probably larger than 100 and smaller than 130, so 115 would be a good
> guess"
> 
> Do you know of any way to implement something like this or are there codes
> doing this?
> 

There are several possibilities,

First you could use RCPC (Rate Compatible Punctured Convolutional) Codes 
by Hagenauer (Digital Object Identifier: 10.1109/26.2763)

Another possibility is to use soft-decision source decoding which 
estimates the current parameter based on the transition probabilities, 
this can be combined with a soft-out channel decoder to estimate a value 
which is close to the one you are trying to decode but still might be 
different. Take a look at
T. Fingscheidt and P. Vary, "Softbit speech decoding: a new approach to 
error concealment", IEEE Trans. Speech Audio Proc. (Digital Object 
Identifier: 10.1109/89.905998)

Further possibilities exist, but besides looking at UEP (based on RCPC 
codes) I suggest you take a look at the Fingscheidt paper.

Regards,
Laurent

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dudelmann wrote:
> Hi all
> 
> I have a question regarding
> channel coding for the wireless channel.
> 
> All the codes I know have error detection and (some) error correction
> capabilities. So far so good. But for audio transmission I don't really
> need to get the exact transmitted values. I don't care if the LSB falls
> from time to time. I do care if a MSB or a sign falls.

Standard solution: encode MSBs with stonger code, and encode LSBs with 
weaker code or don't encode them at all. This can be optimized for any 
given error rate.

> This leads to my
> question:
> 
> Are there channel codes that provide something like a soft decision on the
> transmitted value? Say I transmit an 8 bit value - I transmit the value
> 113. I need a channel code that tells me "there were errors, but the value
> is probably larger than 100 and smaller than 130, so 115 would be a good
> guess"
> 
> Do you know of any way to implement something like this or are there codes
> doing this?

Since your ADPCM data is probably 3 or 4 bits per sample, the best 
solution would be encode entire symbol (or a pack of few symbols) as 
non-uniform QPSK constellation. So the bigger is the error, more bits 
are affected.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

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Steve Pope wrote:

> dudelmann <d...@gmx.de> wrote:
> 
> 
>>I am implementing an ADPCM codec for use in wireless transmissions (as
>>frequent blog visitors may already know :). I have a question regarding
>>channel coding for the wireless channel.
> 
>>Are there channel codes that provide something like a soft decision on the
>>transmitted value?

> 
> Look into UEP (unequal errro protection.  

Nooo.

APDCM is essentially an analog signal: companded residual of LPC. So 
transmit it as analog signal.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

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On 12/17/2009 2:24 AM, dudelmann wrote:
> Hi all
>
> I am implementing an ADPCM codec for use in wireless transmissions (as
> frequent blog visitors may already know :). I have a question regarding
> channel coding for the wireless channel.
>
> All the codes I know have error detection and (some) error correction
> capabilities. So far so good. But for audio transmission I don't really
> need to get the exact transmitted values. I don't care if the LSB falls
> from time to time. I do care if a MSB or a sign falls. This leads to my
> question:
>
> Are there channel codes that provide something like a soft decision on the
> transmitted value? Say I transmit an 8 bit value - I transmit the value
> 113. I need a channel code that tells me "there were errors, but the value
> is probably larger than 100 and smaller than 130, so 115 would be a good
> guess"
>
> Do you know of any way to implement something like this or are there codes
> doing this?
>
> Markus

Maybe I'm missing something, but if you have bits you don't need or 
don't care about you may want to consider not transmitting them.

Usually one way to address this from a system perspective is to apply 
some sort of compression algorithm that makes all the bits transmitted 
of reasonably uniform importance and reclaims the unneeded bandwidth. 
This eliminates the problem of applying UEP and the overhead/complexity 
associated with it.

Just another approach to consider.  If one takes a system approach and 
weighs the complexity/delay/bandwidth/reliability implications of how it 
all works together it sometimes helps to home in on what the best 
solution might be for a particular application given the requirements 
for that application.

-- 
Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.abineau.com

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Vladimir Vassilevsky  <n...@nowhere.com> wrote:

>Steve Pope wrote:

>> dudelmann <d...@gmx.de> wrote:

>>>I am implementing an ADPCM codec for use in wireless transmissions (as
>>>frequent blog visitors may already know :). I have a question regarding

>> 
>>>Are there channel codes that provide something like a soft decision on the
>>>transmitted value?


>> Look into UEP (unequal errro protection.  

>Nooo.

>APDCM is essentially an analog signal: companded residual of LPC. So 
>transmit it as analog signal.

That would work -- analog residual, and digital LPC coefficients.

I can't says that I've ever heard of anyone doing it this way though.

Steve

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Steve Pope wrote:

> Vladimir Vassilevsky  <n...@nowhere.com> wrote:
> 

>>APDCM is essentially an analog signal: companded residual of LPC. So 
>>transmit it as analog signal.
> 
> 
> That would work -- analog residual, and digital LPC coefficients.

LPC is computed from the past. No need to transmit the coefficients.

> I can't says that I've ever heard of anyone doing it this way though.

The concept is similar to that of noise reduction systems with variable 
frequency response, like Dolby-ABC, etc.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

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