SAMPLE FREQUENCY DEFINED
One very noteworthy aspect of digital sample frequency (sample rate) is that it
will define the
of the audio being sampled. This is because the highest
frequency that can be sampled is roughly half of the sampling frequency.
This upper limit is known as the
This discussion requires a minimal understanding of the term
So, please use the link if you are at all unsure of it’s meaning.
We get an idea of how the sampling frequency can affect fidelity with some
Using a frequency response of 20
to 20,000Hz (bandwidth) and a sample
frequency of 48kHz, we can divide the sample rate by a given frequency to
determine just how many times per second that particular frequency is actually
48,000 ÷ 48Hz (Bass Frequency) = 1,000 samples for each peak-to-peak wave.
48,000 ÷ 480Hz (Lo Midrange) = 100 samples for each peak-to-peak wave.
48,000 ÷ 4,800Hz (Upper Midrange) = 10 samples for each peak-to-peak wave.
48,000 ÷ 9,600Hz (High Frequency) = 5 samples for each peak-to-peak wave.
48,000 ÷ 16,000Hz (The upper limit of most quality cassette recorders) =
2.5 samples for each peak-to-peak wave.
48,000 ÷ 20,000Hz (The upper limit of most professional reel-to-reel tape
recorders and human hearing) = 2.4 samples for each peak-to-peak wave.
It becomes readily apparent that the higher the analog audio frequency, the less it
gets sampled. So, what happens to the higher piano notes, cymbals and the like
when lower sample frequencies are used? Obviously, they get sampled very little.
What can happen is that important things like some of the harmonics, which make
up timbre and such, are lost in the sampling process. The resulting digital audio
can sound strident and brittle. The rest of the un-sampled waveform is made up
by the electronics doing the conversion back to analog. Much has been done in
the world of electronic design to counter the problem with playback hardware that
samples the audio signal at much higher sample frequencies than the original
recorded file (over-sampling) so that the re-creation of the higher frequency
content of the audio is more accurate. However, the best of over-sampling
techniques cannot accurately recreate information that wasn’t there in the first
place. So, the higher the sampling frequency during the digitization process, the
more accurate the representation of the analog source will be.
Very low sample frequencies (lower than 32kHz) can result in noticeably poor
fidelity, which is why some answering machines sound as bad as they do.
(not to mention the ‘monotonic’ voice programmed into them)
Because we humans can sense frequencies that are out of our ‘normal’ hearing
range, there are those, particularly in the music industry, who argue in favor of the
higher sample frequencies (96kHz and above).
The higher sampling frequencies provide considerably more bandwidth.
This means that the frequency limiting (Low Pass) filters are well above anything
that humans can perceive.
Common sample frequencies for digital audio
The sample frequency originally specified for the Compact Disk format.
The 44.1kHz sample frequency was chosen because it was the sample frequency
that that worked mathematically with NTSC analog video recorders which, were
the most available device for recording and storing digital audio at the time.
When using a PAL video recorder, (The video standard in Europe) the sampling
frequency is 44.056 kHz.
The sample frequency originally specified for audio playback on DVD.
48kHz is the standard adopted by the feature film industry for all of the digital
sound played in theaters.
Used as an archival format by many who tailor their product for Compact Disk. It
is the opinion of some that, when sample-rate converting to 44.1kHz, the math is
simpler and therefore less data is compromised.
The sample frequency adopted by most institutions worldwide as the archival
The mother of all sample frequencies? We’re talking about being able to sample a
bandwidth up to 96kHz. Now we’re getting into the commercial broadcast range!
But seriously folks, this does get used by those who want to squeeze every
possible sample out of that analog waveform. In fact, some are going beyond
192kHz and sampling at 384kHz! There is even research that suggests that
analog audio should be sampled at 384kHz because we humans can perceive
the difference. Who knows, within 10 years or so, as hardware and storage space
becomes cheaper and faster, it may become the new archival standard.
So, when it comes to creating the digital audio archive file of your family history,
what is the best thing to do?
96kHz is the archival standard that has been adopted by the likes of the Library of
Congress, the European Union, the Audio Engineering Society, and several other
institutions and organizations.
The feature film industry, on the other hand, uses 48kHz as its archival standard
sample frequency. Why the deviation? The prime concern is the fact that 48kHz
files take up half as much storage space as 96kHz files. Today’s lavish film sound
mixes contain hundreds of source files that get archived. It all ads up quickly.
Of all the home recordings that I have had the opportunity to work with, the
average frequency response (bandwidth) of old records has been from around
50Hz to about 5kHz, with the best audio tape recordings having a measured
upper limit of 7 to 12kHz. The truth is that the consumer tape recorders, working
with their supplied microphones and running at much slower speeds than
professional recorders, were rarely capable of producing a bandwidth that
exceeded 12kHz or so.
Thus, a sample frequency of 48kHz will be adequate for the vast majority of home
audio recordings, particularly if storage space is a concern.
As stated above: the higher the sampling frequency, the more accurate the
representation of the analog source will be (storage space considerations aside).
So, in the end, the choice of archival sample frequency is yours. Go with a 48kHz
sample frequency and stay within the (20Hz – 20kHz) audio band, or use 96kHz
which, is the archival standard, for a bandwidth out to about 48kHz, consuming
twice the storage space.
All of the sample frequency standards currently in use will be around for the same
amount of time, so playability in the future should not be an issue.
The other half of the digital audio equation is bit depth. I’ve written an
will hopefully demystify this very important, often misunderstood aspect of digital
audio recording. In the glossaries section is a simple definition of
© Corey Bailey Audio Engineering
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