SAMPLING 101 - part 2 - "Nyquist frequency"

As an definition for Nyquist frequency: in order to reconstruct signal x(t) from its samples, sampling frequency (Fs) needs to be (2x times) higher than bandwidth (B) of the sampled sound (Fs > 2B).

For example, human voice usually contain relatively insignificant frequencies at or above 10 kHz. Sampling such an audio signal with sample rate at 20k samples/sec or more, results an good approximation to meeting the criterion. No problem.

If there is some need (e.g. due to technical limitation or standardisation, to only have 8kHz audio signal), in this case, human voice should be filtered before sampling in order to reduce to aliasing. In this situation the type of filter needed is a lowpass filter (which in this example can be called as anti-aliasing filter.)

Aliasing & anti-aliasing can also be applied in other digital signal processing, e.g. digital image processing.

I'll define aliasing and anti-aliasing later in an another section of this evolving blog.

As a rule of thumb; sampling frequency needs to be (at least) double than the original audio signal's bandwidth.

IT HASN'T ALWAYS BEEN SO EASY - Brief history of audio recorders part 1

Only the multi-track tape recorder enabled meaningful audio editing. Multi-track tape recorders were introduced into the world (courtesy of a company called Ampex in the 1950s). Editing could be done after the recording sessions (kind of copy and paste style with tape), which was very time-consuming work.

Digitalisation started in the 60s with small experiments. However, it was 1981 when CD was introduced to the world. Still digital audio was recorded on tapes (e.g. systems like DASH by Sony and TASCAM).

Still the recording and editing were done with tapes, now as digital signal on the tapes instead of analog signal. One can imagine what kind of 'logistical nightmares' some 70s progressive productions were...

Clear advantages of digital audio (in 80s) were:

• overall noise level was lower
• when adding several tracks all together -> noise level raised less
• quality of the sound remained same (independently of how many times sound was copied & edited)
• consisted sound without classic "wows & wobblings" with analog recordings, which were caused by fluctuations of the tape's speed.

To be continued. Stay tuned!