Question about low bitrate audio, frequencies and number of channels.

[Chainmax]

In the near future, I plan to make low bitrate encodes of some of my DVDs in order to be able to watch them on portable devices. The audio I plan on using in them would be either MP3 (most likely) or Ogg Vorbis (if the device supports it) at bitrates below 96kbps. I'd like to know what frequency and number of channels would you guys recommend me for the following bitrates: 96,64,48,24.

[Kurtnoise]

Originally posted by Chainmax
In the near future, I plan to make low bitrate encodes of some of my DVDs in order to be able to watch them on portable devices. The audio I plan on using in them would be either MP3 (most likely) or Ogg Vorbis (if the device supports it) at bitrates below 96kbps. I'd like to know what frequency and number of channels would you guys recommend me for the following bitrates: 96,64,48,24.

mp3 isn't recommended on such bitrates. A good choice might be Ogg Vorbis for sure with a sampling rate <= 44.1 KHz. But if you have the possibility to switch on other format, AAC+ is really good for such bitrates.

[Chainmax]

I know that MP3 in no way approaches Ogg (and especially HE-AAC) at those bitrates, but AFAIK the devices I'm planning on using these videos on only support MP3. In any case, my question is pretty much format agnostic, isn't it?

[Soulhunter]

Guess a mono mp3 @ 64kbps would still sound acceptable... :o


Bye

[Chainmax]

64kbps would already need mono? At what sample rate, 48Khz? Wouldn't it be better to lower the sample rate and keep the two channels?

[Soulhunter]

Originally posted by Chainmax

64kbps would already need mono?

Imo a stereo mp3 @ 112kbps sounds already "just" acceptable !!!

Think you have to find the right bitrate for ya self...


Originally posted by Chainmax

At what sample rate, 48Khz? Wouldn't it be better to lower the sample rate and keep the two channels?

Afaik most encoders cutting-off the frequencys anyway for low bitrates... :rolleyes:

Anyhow, you could test if downsampling to 32kHz would gain some quality !?!


Bye

[Chainmax]

I know that the encoder will resample automatically, but I'd rather use SSRC and do the downsampling myself.

[hans-jürgen]

Originally posted by Chainmax
64kbps would already need mono? At what sample rate, 48Khz? Wouldn't it be better to lower the sample rate and keep the two channels? Your question isn't easy to answer, because nobody knows your "pain tolerance" which you would have to find out yourself. For example I've seen many people broadcasting in 16 kbps/mono or 32 kbps/stereo MP3 on the Shoutcast/Icecast directories and always wonder why they are doing this.

So you have to do some tests with your portable what sounds acceptable to you. An overall 64 kbps with MP3 is likely to sound better in mono than in stereo, and if you are expecting any kind of high quality ("CD, Transparency, blabla..."), then you shouldn't even use this with music files, but rather only for speech. By the way, LAME downsamples automatically for different settings (using SSRC, as far as I know), so you wouldn't have to choose the sample rate manually.

[Chainmax]

Well, my hearing is quite crappy, but I'd like to end up with something that's at least tolerable for people with good hearing. Also, remember that this is part of a project to encode some of my DVDs, so most of the soundtrack will contain speech. As for the resampling, I'd rather do it myself because that way I can use the following settings:

ssrc.exe --rate 44100 --twopass --dither 1 --pdf 1 input.wav output.wav

that I found while browsing at HydrogenAudio and I don't know if I can get LAME to use it. So, would you recommend mono for 96 kbps encodes too? Also, what sample rates would you recommend me for the four bitrates I stated?

[hans-jürgen]

Originally posted by Chainmax
Well, my hearing is quite crappy, but I'd like to end up with something that's at least tolerable for people with good hearing. Why do you bother for other people if you want to listen on your portable, probably with headphones and/or outside? Does this portable have two speakers or only one so you wouldn't need stereo playback at all if not using headphones?

So, would you recommend mono for 96 kbps encodes too? Also, what sample rates would you recommend me for the four bitrates I stated? You shouldn't care too much for sophisticated SSRC settings (dither etc.) if you want to play the DVD rips on a portable with MP3 or Vorbis at these low bitrates, because they will surely destroy any theoretical advantage from these resampler settings. So I would use the same values that LAME does at these bitrates.

And 96 kbps is the lower limit for stereo MP3 files in my opinion that are supposed to still sound somehow close to the original. But as already mentioned, some people go much lower than that for different purposes.

[Soulhunter]

Btw, what kind of "portable device" we're talking about ???

OT: Chainmax, your pm-box is full... ;)


Bye

[Chainmax]

Why bother for what sounds good to other people? Easy, I am addicted to the warm fuzzy feeling of knowing that if it sounds tolerable to golden ears it will sound good to me :). Also, for bragging rights when I show it to other people.
The portables this would be played on would be the GBA/DS player that has been announced recently (if it comes to the US) and an IBM PocketPC. I will probably use it on a Dreamcast DivX player called uDCDivX as well.


P.S: my PM box is full? Time to go clean it up then :).