Default Output Frequencies

[Tana]

I deal with audio and video encoding for a while but this cought my attention... Whenever I encode WAV with frequency of 48kHz to a smaller bitrate than 128kbps, my frequency is lowered by deafult... for example, 112kbps is 44.1kHz, 96 is 33,6 or so... now, 44.1 is frequency that our ears are used to hear all the time... CD quality is using that frequency... But, what happend if I ignore defaults and encode 96 or 80 kbps in 44.1kHz frequency... It all looks fine to me... The track is pretty good considering that it only has such a small bitrate but it's definitely better than when encoded in default freq... Can anybody tell me if this is bad or it doesn't matter... 10x in advance...

[ultimatebilly]

You're encoding to mp3, right?
Well, for my ears, bitrates below 128 kbit/s doesn't sound good anymore, but it is a matter of taste of course...
The downsampling is done by lame automatically (because fewer samples need fewer bitrate), but there are three problems:

1. Every sample-rate conversion decreases quality (samples have to be repeated or extended, or clipped, depending in which direction you convert the sampling rate)

2. Downsampling so much will remove your high frequencies from the stream (because the sampling frequency must be > 2x the highest frequency in the audio stream - Shannon theorem)...
Human beings can hear frequencies to up to 20000hz, this is the reason the sampling rate for CDs is 441000hz...
Low frequencies only may be fine for spoken language or for telephone conversations, but surely not for high-fidelity movie audio...

3. Depending on what you wanna do with your audio you're restricted to specific sampling rates, mainly being 44100 and 48000...


To prevent lame from downsampling, use --resample 48000... this should work...

[Tana]

Well, this was helpful but... here's the thing... I HAVE TO downsample bitrate to 96 or 80kbps because I want a movie that is 140mins to fit just one CD... The ratio is 2,35:1 and I would also encode it to 512xEqual size... but audio can spare me some bits too... So is it better to encode it to 44.1 because it definitely SOUNDS better or to default freq (33 or even worse, 22)... If I mention that file will have 80kbps and 44.1kHz... Does it sound strange or just... you know... And thanks to "ultimatebilly" for such a fast answer.

[ultimatebilly]

I would suggest using ogg vorbis as the audio format, it can sound quite nice even with bitrates under 90 kbit/s...
Try the oggmachine from http://dspguru.doom9.org

If low-quality audio is okay for you, go ahead, but keep in mind that the viewing pleasure suffers a lot from bad audio...
I would check especially scenes with music and action if it is okay for you...
Generally I would agree that it is better to let lame downsample for very low bitrates...
Since you're doing avi, you're not bound to 441000 or 48000 hz...

If you give ogg-vorbis a try, you will need to mux the audio and the video into an ogm container (which has a smaller overhead than avi, by the way), you can get the necessary filter here: http://tobias.everwicked.com , use oggmux to mux the streams...
If you like to test new things, use the matroska-container, you can mux audio and video to it using virtualdubmod.