Hi David, Thomas,
Thank you very much for your help. I will work on it.
😊
Sumek
________________________________
From: Kurin, Thomas <thomas.ku...@studium.fau.de>
Sent: Wednesday, June 6, 2018 4:46:32 PM
To: freetel-codec2@lists.sourceforge.net
Subject: Re: [Freetel-codec2] noob in voice coding asking for help about Codec2
decoder.
Hi Sumek,
If you want to look at the effect of the postfilter (for Codec 700C) try
to change the postfilter parameters in postfilter.c to Margin 16, Thresh
5 and Beta 0.6 while at the same time changing the pf_gain to 2.5 in
newamp1.c. This lead me to a audible increase in (subjective) quality.
You will definitly hear a change whether you use the postfilter or not.
I'm only working on 700C so I can't really help you with the other
Codecs.
Cheers, Thomas
On 2018-06-05 21:34, David Rowe wrote:
> Hi Sumek,
>
> Wow you are getting into the hard core DSP - well done :-)
>
> 1) Codec 2 uses the sinusoidal model to synthesise the speech, i.e.
> many sine wave oscillators, each with their own amplitude A(m).
>
> So we need to estimate the amplitude of each sine wave from the LPC
> coefficients, which describe an all-pole filter. We do this by
> sampling the all pole LPC spectrum in the frequency domain using
> aks_to_M2().
>
> As you suggest, other synthesis methods are possible (eg time domain
> using the LPC synthesis filter directly) - and would be an interesting
> project for a masters or honors project.
>
> IIRC there is a chapter/section/plot in the thesis that explains how
> we get A(m) from the all pole LPC spectrum by averaging over a band
> rather than direct sampling.
>
> Also some blog posts around 2009:
>
> http://www.rowetel.com/?p=130
>
> BTW I have just added an archive to my blog:
>
> http://www.rowetel.com/?page_id=6172
>
> 2/ I have found the post filter is very important to speech quality,
> here is a blog post:
>
> http://www.rowetel.com/?p=2661
>
> If you install the FreeDV GUI program, and run it in loop back in the
> FreeDV 1600 mode, you adjust the post filter in real time at the top
> of the Tools-Filter menu.
>
> On Codec 2 700C (used in the FreeDV 700D mode), a post filter is the
> key to the (relatively) high quality at such low bit rates. They way
> it works is a little mysterious - and also a good topic for research.
> Thomas, a Masters student recently contacted me and is doing some work
> in this area. He may like to comment :-)
>
> Cheers,
>
> David
>
> On 06/06/18 04:06, sumek.wi via Freetel-codec2 wrote:
>> Hi,
>>
>>
>> I am a noob in voice coding. Now I want to do a project about Codec2,
>> so I went over some
>>
>> theories of voice coding, LPC etc. Also I downloaded the reference
>> C-code and David's Thesis.
>>
>>
>> There are some points I don't understand, I hope you guys can help me
>> on these.
>>
>>
>> 1. In the decoder block, I almost get the idea of how it works (thank
>> to David's PhD thesis)
>>
>> However, I don't understand why after we get LPC
>> coefficients from LSP2LPC process, we
>>
>> have to use FFT (aks_to_M2 function to get ak[i]). As long
>> as I know, LPC coefficients can
>>
>> synthesize speech data by all-pole filter from excitation,
>> I wonder why this algorithm use
>>
>> FFT directly to the ak[i] to get Aw[]. How do LPC
>> coefficients relates to the speech power
>>
>> spectrum. If you guys know any good reference, please let
>> me know.
>>
>> 2. How important is the lpc_post_filter() ? I look at the
>> code, it is pretty high computation
>>
>> process with FFT, and power estimation. I tried to decode
>> with and without post filter at
>>
>> 1200 bps, I don't hear any difference much. what is the
>> criteria to use/not use the post
>>
>> filter ?
>>
>>
>> Thank you for your helps in advance.
>>
>>
>> Sumek.
>>
>>
>>
>>
>>
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>
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