Re: Rip BitRate !!!
- From: cwdjrxyz <spamtrap2@xxxxxxxxxx>
- Date: Wed, 15 Aug 2007 19:39:11 -0700
On Aug 15, 2:50 pm, "zachd [MSFT]"
<za...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
There's lots of data out there about the Red Book Audio CD. I believe it's
about 1400kbps on a CD. But WMA and Red Book use different audio encoding,
so that's rather a poor comparison point. Usually at 192kbps or above the
average person starts not being able to tell the difference on most
equipment on hand.
For the CD PCM standard of 44100 Hz(once called cycles per second)
sample rate and 16-bit(number of bits taken per sample) , the total
bit rate is 44100 x 16 = 705600 bps = 705.6 kbps per channel. This
would apply to a single channel mono recording. For the more usual
stereo CD with 2 channels, the above has to be multiplied by the
number of channels used, in this case 2, to give 1411 kbps. You need
to know both the frequency(Hz) and number of bits taken per sample.
This is what usually is specified for a CD, and, as shown, the total
bit rate can be derived from these 2 numbers and knowing the number of
channels recorded.
It is quite easy to use an uncompressed PCM CD standard audio file on
the web, since high broadband now will easily pass such a signal in
real time. Using ATT/Yahoo Elite DSL, I can handle signals up to about
5 Mbps, and 1.4 Mbps for PCM is very easy. The main reason for using a
compressed audio format on the web these days is to allow dialup users
and low broadband users to hear the audio in real time without undue
buffering time delay. Bandwidth has become dirt cheap if you find the
right host that often serves small business, so increased bandwidth
usually is not a factor anymore for using PCM for private web pages
that use a little music now and then. I have quite a bit of audio and
video on my 2 domains. I also have a domain for the FAQs of a Usenet
group put up on my host account. However I never come close to
exceeding the 500 GB bandwidth limit per month for which I have paid.
Two of the best sound sources for evaluation of the sound of
compressed media are applause and falling water. These sounds tend to
give compressed formats the most problems. The 2 sounds to be compared
must be exactly matched in volume, as otherwise people tend to select
the loudest. With a proper controlled blind test with the reference
being the PCM wave and the test signal being a compressed format, it
is found that a mp3 needs to be at least about 300 kbps so that the
reference and compressed format sound can not be told apart using
outstanding applause or falling water recordings. Of course if you
listen with an inexpensive portable device with cheap headphones, you
can get away with a much lower bit rate so long as you do not plan to
play the recordings back on a high end audio system.
Another consideration is the dynamic range of the music and noise
floor. This is more of a consideration for some classical music than
for most pop/rock music. An extreme example would be an uncompressed
recording of the Berlioz Requiem. The sound level ranges from a
whisper that can barely be heard in a very quiet room to perhaps a 100
piece orchestra, 4 brass bands in the corners of the hall, a large
chorus, and several solo singers, all playing and singing at once as
loud as they can. Even standard CD PCM recording is borderline for
such an extreme examples. Thus DVD-Audio and SACD were introduced to
lower the noise floor and increase the dynamic range, among other
things. Both of these formats record at higher frequency(Hz) and bit
rate than the PCM CD standard.
.
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- From: zachd [MSFT]
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