| From: | Martin Nicholson |
| Date: | 4 Aug 2000 at 21:23:16 |
| Subject: | Re: Mpeg 3 playback. |
Hiya Andrew
On 04-Aug-00, Andrew Crowe wrote:
> Hi Everybody,
>
>>>> Anyway, even with an 060 or a PPC you'd be wasting your time, if
>>>> you wanted the full quality of the sound.
>>>
>>> You jest! A PPC easily copes with full-quality decoding of MP3. A
>>> 604/233 can *encode* faster than realtime.
>>
>> Not at all. Has I said if you want the *full* quality! Mp3s are
>> encoded as 16 bit, not 8 bit or even pseudo-14 bit. So even if you
>> could put a G3 PPC to work, it still squirts out through an 8 bit
>> sound chip.
>
> And if you had a sound card?
Then you'd be able to output in 16 bit. If you had a fast processor
that could decode the mpeg stream at it's full sampling rate, then a
soundcard is all you need. If not, then an on-board mpeg decoder would
be nice :) Actually, it'd be nice regardless of CPU.
> Anyway, pseudo-14 bit sounds perfectly fine to me anyway.
It does a pretty good job, but I think I'm right in saying that it
also steals CPU cycles from you. Basically though, at the end of the
day, pseudo 14 bit sound from an 8 bit sound chip is not going to be
on a par with that from a 16 bit or greater soundcard.
> By the way, are you sure that mp3's are saved as 16 bit, or any
> number of bits for that matter? I'm not sure, but I have a feeling
> it saves waveform data, not the actualy wave (like instead of saving
> a wave like a sample, it saves it as a frequency and volume or
> sommin.) If that's true, then in theory a decoder could decode at an
> infinite bit res.
I don't pretend to understand the intricacies of how an mp3 data
stream is built up. But since, in the case of CD audio anyway, it is
built from a 16 bit wave, and since it outputs a 16 bit waveform, how
it actually stores that waveform is immaterial. As to infinite bit
resolution, what would be the point?? As far as I Know (and I'm
probably wrong, some of this is just adding 2 and 2 to make 5), if a waveform is in 8 bit, then it has 256 points along the amplitude of the
waveform (-128 to 127), whereas if it's in 16 bit, it has 65536 points
(-32768 to 32767). The higher the bit resolution is, then the less
quantisation errors will occur when an analogue signal is being
sampled into a digital form.
Thus, if you decoded a 16 bit sample at a higher bitrate, then surely
you'd just "pick up holes", i.e., at some points, you'd be decoding
white noise, because there'd be no signal there? Besides, if the
pundits are correct (and to be honest I don't think they are) only the
audio signal between 20Hz and 20KHz is important, because they are the
audible limits that the human ear imposes. Thus 44100 KHz is about as
high a samplerate as you need, and this is adequately provided for by
a 16 bit sample rate. Of course, you could listen to Vinyl and then
you would get an infinite sample rate, as well as all those inaudible
frequencies below 20Hz and above 20KHz (which is why vinyl sounds
better ;-) )
Whats more important, at least so it seems to me, is the bitrate of
the mp3, i.e., whether it is 96, 128, 192, etc, KBPS. This, I think, is
the rate at which the mp3 encoding is sampling the waveform of the
wave sample. So that the mp3 isn't actually a 16 bit sample, so much
as a bit rate, i.e., sampled so many times a second (kpbs)? The
greater the kbps is, the bigger the sample will also be. An infinite
bit rate would generate a massive file size.
Ah well, you can all tell me where I'm wrong now :)
Oh, and just to clarify, I'm not knocking the Amiga's sound. I
originally was trying to point out that, by and large, accepting a
lower frequency and stereo, was possibly better than a higher frequency
and mono, because most Mp3s are in stereo and since mpega outputs the
left channel in mono, you may find the music sounds a little strange
at times :))
With Regards
Worzel
Nick Worzel on #IRC AlternativeNET
ICQ UIN 14617500
mailto:martin@madscientist.demon.co.uk
I used to work in a fire hydrant factory. You couldn't park anywhere near the place.
Quote carefully and read all ADMIN:README mails