How does bitrate affect sound fidelity?

[Xoanon]

Using DVD soundtracks as an example, I understand that a higher bitrate means better sound quality (comparing, say, 192kbps to 448kbps), but why is this technically? What is a bitrate, and whats is meant by "bits per second"? Is this how many "bits" of data are processed/decoded or something?
Or is it something to do with frequencies? I mean after all, 48kHz is going to sound better than 44.1kHz, so in theory frequency should only affected the sound quality, when obviously it doesnt because the bitrate has an effect too.

Ive just never stopped to think about it, sorry if it sounds like a newbie post - its not, lol! I just hope I've been clear enough.

[lordsmurf]

Bitrate is amount of data used. Less is less quality. In fact, bitrate affects quality FAR MORE than frequency alone.

But there are quality plateaus for both: Anything beyond 256k for 48kHz stereo overkill (AC3, MP2). Even 192k and 224k are fine most of the time.

Generally, your 44.1kHz MP3 is great at 128k-160k for music.

Also, codecs has some control. MP2, MP3, AC3, WAV, DTS, all different animals.

Complex topic, no simple answer, no "one size fits all" answer.

[Xoanon]

Bitrate is amount of data used

OK, so a bitrate is [x]kbps. What is meant by the "per second" bit? Is that how many bits are being heard/decoded/encoded/something else?

[Xoanon]

BTW, in this instance Im actuallt referring to DVD AC3 and DTS tracks

[SLK001]

196kbps second is just that - 196k bits consumed every second.

[Xoanon]

Consumed? Is that space on the disc being consumed? And if so, how does using more disc space contribute to better sound? Or am I completely wrong, lol?

[lordsmurf]

Completely ... off base.

Compression is the nature of the game. Less bits, harsher compression. Harsher compression, worse sound.

[Xoanon]

Is an audio bitrate "bit" the same as any normal data "bit"? Like a byte?

[Xoanon]

And sorry, whats being compressed, the frequencies? I thought that was separate?

[redwudz]

I found this site that might explain more about digital audio, bitrates and sampling rates. Sorry, it's a little involved, but most of your questions should be answered there.http://www.teamcombooks.com/mp3handbook/11.htm

[Xoanon]

Thanks, will check it out.

[MOVIEGEEK]

And sorry, whats being compressed, the frequencies? I thought that was separate?

Yes to a certain extent,the more you compress the more frequency range you lose(usually the top end).AC3 and DTS have a wider dynamic range than MP2 and retain their frequency range better.
Here is a list of audio quality(best to worst):
1.LPCM
2.DTS
3.AC3
4.MP2

[Xoanon]

But frequencies are handled by whether a file is 48kHz or whatever, right? Man this is all confusing lol!

Oh and I also read a site that said AC3 was better than DTS - why is nowt straightfoward?! :P

[MOVIEGEEK]

At any given bitrate DTS will always be better,whether you can hear the differance is debatable(I can).
Samplerate is basically the frequency range:
44.1KHz(stereo) will have a freq. range of ~20-22,050Hz.
48KHz(stereo) will be ~20-24,000Hz.
For a reference:
AM=100-10,000Hz
FM=50-15,000Hz
Cassette=~100-12,000Hz
Cassette(metal)=~100-15,000Hz

Bit Depth is the amount of info. packed into every byte of data:
16=CD/DVD-VIDEO quality
24=DVD-AUDIO

[Xoanon]

Heavy stuff, lol.

OK. So, sample rate covers the frequency range. OK, thats fine. But how does bitrate tie in with frequency and how do lower bitrates lose higher frequencies when frequency is handled by sample rate? If that makes sense.

For someone who regularly masters DVDs with 5.1 tracks, etc. this is quite scary to see how little I know lol. All I know is higher bitrates equal better sound, so I just do my DVDs at 448kbps AC3.

And why does the DVD specs not allow anything above 448kbps?

[MOVIEGEEK]

1.When you compress audio you have to lose something so encoders do away with redundant data or data that isn't needed such as frequencies >16KHz.The same when you transcode video,some data is removed from the GOP.
2.I'm not sure why DVD specs don't allow higher bitrates,it's possible Dolby limits it.

[Xoanon]

Am I right in saying the frequencies above 16kHz are those we cannot hear?

[MOVIEGEEK]

Most adults can't hear above 16KHz.
You can try this yourself:
1.Download Audacity.
2.In Generate->Tone select Sine and produce several freq. clips ~15 seconds long.
3.Export to WAV and burn an audio CD.
4.Play in your car/home stereo or PC with headphones*
*most PC speakers only go to 12KHz

[Xoanon]

What will this CD prove? That we cant hear above that frequency?

[Jester700]

At any given bitrate DTS will always be better,whether you can hear the differance is debatable(I can).

Most of your post contained good guidelines, but this is misleading at best, probably wrong, and impossible to prove in any case.

The mastering applied to AC3 and DTS are usually different, as are the average levels AND the usual bitrates - so although many people argue this point, few have done fair listening tests, or even have the ability to.

And generally when someone says "this codec is best at all bitrates" they're wrong - most codecs were designed & tweaked with a certain bitrate or range of bitrates in mind, though they can be used with a much wider range. In the 256k range, the older MP2 is considered by many to be superior even to the newer & flashier ones (like WMA9 & others). Why? It's simpler - this allows less compressability but fewer artifacts when the bitrate supports it. But at 64k WMA and Real walk all over MP2 and MP3, because that's what they were primarily designed for.

[MACCA350]

You cant directly compare DD & DTS , as they use different technology, and are mastered differently (eg. its well known that DTS boost their bass by atleast 3db among other things). Also you cant compare the bitrates directly as DD does better at the same bitrate, which is part of why DTS uses higher bitrates.

I also read somewhere that dolby uses a normalisation featurewhere receivers will trim the volume depending on what dolby signal is received and this has the effect of lowering a Dolby Digital 5.1/EX signal by 4db ( I think this is on dolby or THX website someware in the tech papers)

Another artical "Dolby Evaluates DTS" part 1&2 maybe a bias report but shows that DD maybe closer to the master than DTS, and DTS sweetens the mix etc. As expected DTS Deny this.

Eitherway its not that easy to compare the two, or others for that matter. I would love to see LPCM (uncompressed PCM) Multi-channel at 24bit/192kHz (much higher dynamic 140db and frequency 40kHz+ range)used for audio in the new HD-DVD or Blu-ray high definition formats for movies.

in the end its up to the indervidual and what sounds better to them.

cheers