[pulseaudio-discuss] LFE remixing

[Alexander E. Patrakov]

06.11.2014 15:39, David Henningsson wrote:
>
>
> On 2014-09-30 15:59, Alexander E. Patrakov wrote:
>> 30.09.2014 19:07, David Henningsson wrote:
>>>
>>>
>>> On 2014-09-26 14:17, Tanu Kaskinen wrote:
>>>> A summary of my proposal:
>>>>
>>>> To add LFE low-pass filtering, just hack the current remixer
>>>> implementation, no need for interface changes anywhere.
>>>
>>> Actually, I think this could be good enough, at least as a first step.
>>>
>>> Do you think it would be okay to:
>>>
>>>   - Implement a LR2 or LR4 filter by "hacking the current remixer
>>> implementation"
>>
>> For me, it would be okay, but I would prefer a slightly more complex
>> plan, purely to avoid the blame of the form "you demand fully rewindable
>> code everywhere from others but just wrote an LFE-remixer that doesn't
>> handle rewinds correctly". The plan is:
>>
>> 1. Implement functions to calculate the coefficients of the required IIR
>> filters for LR2 and/or LR4.
>>
>> 2. Implement, in a separate file, some set of functions that would allow
>> one to apply an arbitrary matrix of IIR filters on a memblockq. Make
>> sure they are fully rewindable (that's the PR part), write a test.
>>
>> 3. Connect functions from (2) to the existing remixer implementation. If
>> it turns out that it is impossible to form the rewind requests
>> correctly, write a TODO item.
>>
>> (2), except the test, already sits on my laptop since the previous year
>> in the form of a virtual sink. I just need to refactor it from the
>> "parallel biquads" to the "chain of biquads" form (because both LR2 and
>> LR4 have pairs of identical poles which are inexpressible in the
>> parallel-biquads formulation) and remove the sink code.
>
> Reviving this thread due to renewed interest from management. In short,
> we should probably go ahead and write this filter. I can certainly try
> to do it, but I assume you will just complain about me breaking the
> math. :-)

That's OK.

> Did you end up doing anything more with your existing code? Need a hand
> anywhere?

No. In fact, I have not looked at PulseAudio code (except Peter's 
patches) since the conference, and I don't have enough free time. The 
earliest date when I can continue is this Sunday.

-- 
Alexander E. Patrakov