AC3 - is low pass filter needed at 640k?

[mindbomb]

Hi, I have a question about the Dolby AC3 Encoder.

It gives options for preprocessing a bandwidth low pass filter.

If I am compressing 6 channel lossless audio from a bluray at 640k, do I need any of these features?

I am thinking since it is 640k, the bandwidth low pass filters isn't needed.

edit:
I'm being told the bandwidth low pass filter is still good to have.

[tebasuna51]

You don't need preprocessing filters to convert audio from BD.

[SquallMX]

Official Dolby Encoder cuts all frequencies above 20.31 Khz regardless of the bitrate, Aften AC3 encoder does not (keeps up to 24 Khz).

[mindbomb]

wait, so what is the point of the preprocessing low pass filter if the encoder will throw away the high frequencies anyway?

the cutoff point is 20.34 khz btw.

[LoRd_MuldeR]

wait, so what is the point of the preprocessing low pass filter if the encoder will throw away the high frequencies anyway?

The lowpass filter is what "throws anyway" high frequencies. Though this doesn't mean that any encoder will necessarily apply a lowpass filter.

(Often the lowpass filter is invoked depending on the selected target bitrate)

[amtm]

I think the point of their question is why does it have a low-pass filter option if it's going to run the low-pass filter no matter if you select it or not. So his question is what does the option do extra if the encoder is already low-passing anyway.

[LoRd_MuldeR]

Well, if the encoder offers "lowpass filter" as an option, then it obviously should not be using a lowpass filter, if you explicitly disable that option.

(The only other explanation that I can think of is that this option can be used to enforce the lowpass filter in situations where the encoder normally wouldn't use it. But then it would make much more sense to offer three values for the lowpass filter option: Enabled, Disabled and Undefined/Auto)

[amtm]

Obviously. That was why he was trying to get the behavior clarified in light of SquallMX's post saying it always low-passes.

[SquallMX]

Original Stereo WAV File (48 Khz/16 bits from lossless DTS-HD MA Source):
http://img824.imageshack.us/img824/2272/originalwe.png

AC3 File Encoded by Aften (320 Kbps):
http://img5.imageshack.us/img5/3518/aftenc.png

AC3 File Encoded by Sound Forge Pro AC3 Plug-in (320 Kbs, No Lowpass):
http://img694.imageshack.us/img694/1549/dolbynolowpass.png

AC3 File Encoded by Sound Forge Pro AC3 Plug-in (320 Kbs, Lowpass box checked):
http://img535.imageshack.us/img535/5407/dolbylowpass.png

So... no bandwidth low pass filter = 20.40~ Khz Frecuency Cut, bandwidth low pass filter selected = 20~ Khz Frecuency Cut???