FPGA Central

Hi all

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

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

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

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

Markus

dudelmann <[email protected]> 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

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

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

Markus

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

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

Steve Pope wrote:

> dudelmann <[email protected]> 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

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

Vladimir Vassilevsky <[email protected]> wrote:

>Steve Pope wrote:

>> dudelmann <[email protected]> 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

Steve Pope wrote:

> Vladimir Vassilevsky <[email protected]> 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