[pulseaudio-discuss] [PATCH] Add option to disable highpass-filter on non-lfe channels.

[Markus Ebner]

On 08/11/2017 06:51 PM, Alexander E. Patrakov wrote:
> 2017-08-11 21:48 GMT+05:00 Alexander E. Patrakov <patrakov at gmail.com>:
>> 2017-08-10 1:27 GMT+05:00 Markus Ebner <markus-ebner at web.de>:
>>> Added a new configuration option: remix-non-lfe-channels (default: yes), which results in the non-lfe channels being highpass-filtered.
>>> When set to false, the lfe channel is lowpass-filtered and the other channels stay untouched.
>>> Removing the low frequencies from speakers that support low frequencies sounds awful.
>>> The current behavior is the default, since this case is less likely than speakers not playing well with low frequencies.
>> I tend to reject the patch. In the use case with satellite speakers
>> that are able to reproduce low frequencies, enabling LFE remixing (and
>> thus the filter) is a user error.
> Also, the highpass filter on non-lfe channels was designed to filter
> out exactly the same frequencies that were sent from the main channels
> to LFE. I.e. "moved". With your patch, they will be "duplicated",
> which is wrong.
>
>
I'm not that knowledgeable regarding audio, so my reply represents what 
I think I know of the subject.

I am not quite sure what the problem is. In the Computer Graphics field, 
the simple principle is: All's well that looks well.
I thought that the same applied to audio. As long as it sounds good, why 
bother with the details?

Most "cheap" 5.1 computer speakers exist of
- 4 satellites (high to maybe center frequencies)
- 1 center (center frequencies)
- 1 subwoofer (low frequenciesa)

Pulseaudio's lfe remix in its current form does exactly what is best for 
this setup, since only the subwoofer is able to represent low frequencies.

Whereas the satellites of high-class speaker-setups (like the "Teufel 
Theater 500 Surround 5.1 Set") support the full spectrum of frequencies.
The satellite of the above mentioned setup ("Tower Speaker T 500 F 16") 
has dedicated woofers inside.
And if I haven't made a mistake, driving this setup with the lfe-remix 
enabled will result in the towers' woofers to effectively be disabled.
Why would I pay 1700€ for a high-class setup if I can't properly drive 
its speakers?

Sure, this particular setup probably has an analog frequency-separation 
filter inside the subwoofer, but there are setups that haven't. In such 
a setup (without filter), disabling the lfe-remixer will lead to the 
subwoofer playing the whole frequency spectrum - which isn't that good 
of an idea. And not all analog filters are better than pulseaudios' (We 
tested a couple).

Our particular setup is:
- 2 x the box CL 110 Top MK II (90 - 19000Hz)
- 1 x custom made subwoofer (without an analog frequency filter inside) 
(20 - 140Hz)
Comparing the unpatched pulseaudio (lfe remix on/off) does not sound 
good, while with the patch, it sounds much better (on).