Hi all,
For the development of the 2400 bps modem, we also need a FEC system
that goes with it.
Please find below a proposal for this.
If somebody has any ideas or remarks about this, feel free to reply.
Concidering,
* the "production" codec2 gmsk modem specification is based on a number
of different "profiles". These are "typical usage" senarios which match
a certain choice of features provided by them mode. The goal is to
reduce the complexity of the modem as it only needs to implement the
features specified for this particular profile.
Current profiles are "local communication" and "DX".
* The "local communication" profile is designed for -as its name
implies- local communication and has the least complexity. One of the
elements of this profile is that everything related for FEC (forward
error correction) and interleaving (changing the order of the bits to
make it less vulnerable to burst errors) is only applied INSIDE the 40
ms timeslot.
* For reasons of syncronisation, a 24 bits "sync/type-of-frame" pattern
has to be inserted into the stream and be repeated. This is send at
least once per eight 40ms timeslots. If no sync/type-of-frame is send,
the space can be used for voice (in this case, voice FEC).
Future options of the specification will provide the possibility to
encapsulate data into that place of the stream. However, the current
version of the specification does not offer this possibility.
* A 40 ms timeslot for a 2400 bps modem is equivalent to 96 bits. The 96
bit structure depends on two factors:
-> Is there a sync/type-of-frame pattern present in that timeslot
-> Is the 1200 or 1400 bps voice codec used?
The structure can be found further down this document.
* A codec2 frame is based on a 40 ms timeslot, containing information
about four 10 ms frames. It has the following structure:
1/ For the 1400 bps voice codec
- 1 "voiced" bit for frame 0
- 1 "voiced" bit for frame 1
- 8 "energy or WoE" bits for frames 0 and 1
- 1 "voiced" bit for frame 2
- 1 "voiced" bit for frame 3
- 8 "energy or WoE" bits for frames 2 and 3
- 36 "LSP" bits for frames 0 up to 3
Total: 56 bits
The 36 LSP bits represent 10 values, using between 2 and 4 bits for
every values.
2/ For the 1200 bps voice codec
- 1 "voiced" bit for frame 0
- 1 "voiced" bit for frame 1
- 8 "energy+wo" bits for frames 0 and 1
- 1 "voiced" bit for frame 2
- 1 "voiced" bit for frame 3
- 8 "energy+wo" bits for frames 2 and 3
- 27 "LSP" bits for frames 0 up to 3
- 1 "spare" bit
Total: 48 bits
The 27 bits "LSP" bits can be broken down into 3 x 9-bit VQ indexes.
* Not all information in the codec2 voice frame is equaly important.
Corrupting certain bits produces more audiable error then other bits.
The importance of the different groups of bits is as follows (in
descending order):
- voicing bits
- energy+Wo bits
- MSB part of the LSPs
- remaining part of the LSPs
- spare bit
* The current FEC systems can be applied:
- golay(24,12,8): this uses 12 FEC bits to protect 12 data bits; with
the ability to correct up to 3 bits and detect up to 7 bit errors
(see http://en.wikipedia.org/wiki/Binary_Golay_code)
- hamming(8,4,4): this uses 4 FEC bits to protect 4 data bits; with the
ability to correct up to 1 bit and detect up to 2 bit errors
(see
http://en.wikipedia.org/wiki/Hamming_code#Hamming.287.2C4.29_code_with_an_additional_parity_bit)
These FEC systems are chosen because they operate a relative small block
of bits (as all FEC is done inside a 40 ms timeframe), free of patents
and opensource source code for it is available.
As a result, the following 96 bit frame-structures are defined:
1/ for the 1200 bps voice codec:
1a/ for a timeslot WITHOUT sync/type-of-frame pattern:
- 48 bits codec2 voice
- 48 bits FEC
In this senario, four times golay(24,12,8) is applied on the complete
codec2 frame.
1b/ for a timeslot WITH sync/type-of-frame pattern:
- 24 bits sync/type-of-frame
- 48 bits codec2 voice
- 24 bits FEC
In this senario, two times golay(24,12,8) is applied, with the ability
to protect 24 bits. The following bits are be protected:
- all 4 voiced bits
- all 16 energy+wo bits
- 4 MSB bits of LSPs (Which ones? Is there a preference for certain LSPs).
2/ for the 1400 bps voice codec:
2a/ for a timeslot WITHOUT sync/type-of-frame pattern:
- 56 bits codec2 voice
- 40 bits FEC,
In this senario,
. three times golay(24,12,8) is applied, with the ability to protect 36
bits.
. in addition, once hamming(8,4,4) is applied, with the ability to
protect 4 bits
The following bits are be protected
- all 4 voiced bits
- all 16 energy+wo bits
- 20 MSB bits of LSPs (2 bits for every of the 10 values?)
2b/ For a timeslot WITH sync/type-of-frame pattern:
- 24 bits sync/type-of-frame
- 56 bits codec2 voice
- 16 bits FEC
In this senario,
- once golay(24,12,8) is applied, with the ability to protect 12 bits
- in addition, one hamming(8,4,4) is applied, with the ability to
protect 4 bits.
Firstly, the golay(24,12,8) is applied to the following bits:
- all 4 voiced bits
- the 8 energy/wo bits that apply to the first two codec2 frames of the
(4 frame) codec2 timeslot
For the 4 remaing FEC bits (the hamming(8,4,4) block), I would like to
propose this:
The hamming(8,4,4) is applied NOT 4 of the other eight energy+wo bits
(for the 3th and 4th frame), but 4 of the MSBs of the LSPs.
The reason for this is as follows:
- the golay FEC decoding not only provides the ability to correct
errors; but also check on the amount of errors found.
- This means that the decoding process has knowledge of the quality of
the link
- If the decoding noticed that the quality is bad (high BER); the FEC
decoder can decide that -as the 3th and 4th frame (where the enery+wo
bits are not well protected) are going to be corrupted anyway- it will
ignore these last frames frames and put a copy of first two frames
(which do have FEC applied) in these frames.
Based on that, it makes more sence to use the 4 FEC bits of the
additional hamming(8,4) block to protect a part of the LSP-information
as this applies for all 4 frames of the 40 ms timeslot.
Please let me know if you have any remarks or questions.
73 - Kristoff - ON1ARF
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