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 How
do I compress audio using QuickTime?
To compress audio you will need the QuickTime Pro Player from
Apple Computer, or any other application which
supports QuickTime export in order to encode audio. If you are using
QuickTime Pro please follow the instructions found below. If you are using
another application please follow the instructions that accompany the software.
 How
do I use the QDesign Music Codec to compress my audio files?
Open an uncompressed audio file with QuickTime Pro. Go to the File menu
and select Export... Export As should be set to either Sound to AIFF or
Movie to QuickTime Movie. Press the Options... button. If you have chosen
Sound to AIFF as your export type then skip to step 6. If you have chosen
Movie to QuickTime Movie then you need to press Settings... in the Sound
area. Set the Compressor to "QDesign Music Codec". Set the desired Rate,
and Use. The Size must remain "16 bit". Press the Options... button. Select
the desired "Bitrate". Save the file.
What
sample rate should I use?
Normally, you should first encode the audio at the sample rate of the
original file (i.e. 44.1 KHz for audio recorded from CD), and listen to
the results. Most of the time this will be the best choice. Depending
on the target bitrate, and for some audio material (e.g. speech or other
band-limited source material), encoding at half of the original sample
rate (i.e. 22.05 KHz is half of 44.1 KHz) will result in a noticeable
improvement in quality. Try to avoid encoding at sample rates which would
result in non-integral sample rate conversion on playback. For example,
if your target playback platform is Mac computers with only 44.1 kHz playback
capability, you should encode your audio at 44.1 kHz or 22.05 kHz sample
rate to avoid a loss of quality from non-integral sample rate conversion
on playback. (for example, encoding at 8 kHz sample rate and playing back
(sample rate converted) at 44.1 kHz is normally a bad idea)
How
do I make a QuickTime Movie file encoded with the QDesign Music Codec
ready for streaming on the Internet?
Begin encoding a file as outlined above in "How do I use
the QDesign Music Codec to compress my audio files?". When you reach
step 5 of the question "How do I use the QDesign Music
Codec to compress my audio files?", move to step 3 below. Below the
Sound area are two check boxes, "Fast Start for the Internet" and "Compress
Movie Header". Check both boxes. Continue with the encoding process in
step 5 of the above question "How do I use the QDesign
Music Codec to compress my audio files?".
What
is kbits/s?
This stands for 'kilobits per second' and is a measure of data throughput
(bitrate) in thousands of bits per second. Your '28.8' modem is potentially
capable of transmitting 28.8 kbits/s. A 56 kbits/s modem can transmit
twice as much data, depending on the quality of the telephone connection.
As a point of reference, the data rate of a stereo track on a CD is over
1.4 Mbits/s (1.4 million bits per second) or nearly 50 times the data
rate of a 28.8 modem.
Which
bitrate should I use?
The highest bitrate possible for your application should always be used.
For example, to produce audio that streams over a 28.8 Kbps modem connection
you should use a bitrate of 20 or 24 kbits/s. For higher quality music download or video applications,
a bitrate of 64 to 128 kbits/s might be more appropriate.
What
steps should I take to get the optimum performance out of the encoder?
In general, use only high quality, digitally-recorded audio source material.
If you are encoding material from a CD, try to record the file via direct
digital transfer instead of recording through the analog input of your
system's sound card. This can be done with standard tools such as QuickTime
4 Pro on the Mac, and with both commercial products and shareware tools
on the PC. If you must record from an analog audio source, use the highest
quality sound card available. The garbage-in, garbage-out principle applies
as much to audio coding as to anything else. Ensure that input levels
are set appropriately and that the input signal is not 'clipping' (hitting
full scale signal strength) by checking the level meter associated with
the input volume control (PC), external mixer, or audio recording application.
A good setting for the input signal level is approximately 2 dB below
full input load level (below clipping). Retaining audio quality at very
low bitrates involves making trade-offs. Try to encode at the highest
possible bitrate for your application. For choice of sample rate, start
with the coded sample rate equal to the sample rate of the source file,
but if coding artifacts are audible, reduce the sample rate to half of
the source sample rate. If artifacts are still audible, try encoding the
file as mono. This should allow nearly a factor of two reduction in bitrate
compared with the stereo file.
What
should I do to get optimal performance out of the decoder?
For best results, ensure that your QuickTime sample rate settings are
set to the same sample rate as the audio file you are playing back. This
is done from the Control Panel/Sound /Sound Out dialog settings on the
Macintosh, and the Control Panel/QuickTime/Sound Out settings dialog box
on Windows platforms. If the sound card playback sample rate is set differently
from the encoded file sample rate, additional sample rate conversion will
be performed by QuickTime, resulting in a loss of audio fidelity.
Why
does the QDesign Music Codec perform so much better than other codecs
at low bitrates?
1.The QDesign Music Codec takes a fundamentally new approach to
encoding audio for low bitrate applications. Older technology, psychoacoustic
codecs (such as MPEG, AC-3) analyze the audio through static filterbanks
or transforms, which are used to approximate the characteristics of the
human auditory system. These systems were developed to minimize the bitrate
required to preserve 'transparent' audio quality, and were not designed
to function below the threshold where coding artifacts become audible.
2. Although the QDesign Music Codec employs well-known psychoacoustic
principles in the analysis, it also builds a source model of the signal,
separating the signal into perceptually-relevant 'audio building blocks'
and developing a parametric representation which can be efficiently encoded
for transmission or storage.
3. This joint source and receiver model allows the QDesign Music
Codec to minimize audible distortion well below the bitrates previously
attained by codecs employing psychoacoustic-based receiver models alone.
Because the encoding process was developed from the 'ground up' the algorithm
is particularly well suited to maintaining high quality, wide bandwidth
signals at even bitrates of 1 bit per sample per second or lower.
I
can't encode using the QDesign Music Codec!
If you are using a Macintosh, it is possible that there is an extension
conflict. Reboot the machine with system extensions only and then try
encoding. Make sure that you are exporting the file as an AIFF or QuickTime
Movie file. The QDesign Music Codec may only be used to create AIFF or
QuickTime Movie files.
I
export a file using the QDesign Music Codec, and nothing happens for a
very long time. Am I doing something wrong?
You're not doing anything wrong. This can occur when exporting AIFF files.
QuickTime does not pop up the progress bar dialog until a certain percentage
of the way through the file, and for long AIFF files this can be a long
time. The codec is still operating even though progress is not being displayed.
This is not a problem when working with Movie files, as the progress bar
is displayed much faster when exporting Movie (.MOV) files than when exporting
to AIFF (.AIF) files.
What
is the difference between the Basic and Professional Editions of the QDesign
Music Codec?
The main difference between the Professional Edition and the Basic Edition of the
QDesign Music Codec is in the level of
control over compression ratio and quality. The Basic Edition offers good
quality audio encoding at selected bitrates. The Professional Edition
offers advanced perceptual coding parameters that allow the user to further
optimize audio fidelity depending on the audio type (i.e., speech, orchestral
music, male/female vocals, etc.), access to an extended range of bitrates,
pre-processing controls, faster compression performance and support
for batch encoding. Read more about the QDesign
Music Codec
How
do I get the QDesign Music Codec Professional Edition?
The Basic Edition of the QDesign Music Codec
is shipped as the standard music codec for Apple QuickTime. The Professional
Edition of the QDesign Music Codec is available for purchase directly
through QDesign Corporation or authorized resellers.
Is
the QDesign Music technology available for licensing so I can include
it into my application?
Yes. The QDesign Music technology is available for licensing directly
from QDesign Corporation. Please contact us for more details.
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