Download Record and Playback

Record and Playback
Introduction
Using digital signal processing, it is a straightforward business to record and playback.
This application shows how to use buffers for record and playback and illustrates the DSP
concepts of decimation and interpolation.
Recording
The TMS320C6711 DSK with the Audio Daughter Card can record the input from CD
player or microphone in one of two ways. Normal recording can be used for recording a
signal from a CD player. The buffer is continually overwritten with new values until
recording is halted manually. Voice activated recording is used with a microphone input
and will start recording when the input level exceeds a pre-determined level. Recording
stops a short time after the input level drops below the pre-determined level..
Continuous Playback
We can record a section of audio and continually play it back in a loop. This involves
continually going around the buffer. A practical application for this is for musicians
trying to transcribe note-for-note a solo or other passage from an original recording.
Double-Speed Playback
To play back at double-speed can be achieved by ignoring every other recorded input
sample. This process is referred to as decimation. There is however, some loss of quality.
In musical terms, this raises the pitch of the audio signal by an octave.
This can be useful for transcribing bass guitar. By raising the pitch an octave, the
individual notes of the bass guitar can be more clearly heard.
Half-Speed Playback
In order to play back at half speed, either the sampling rate on playback can be decreased
or the sampling rate kept the same and intermediate samples generated. A simple scheme
Tutorial 3: Record and Playback
14 May, 2017
for generating extra samples is to take the average of the samples either side using
interpolation. In musical terms, this lowers the pitch of the note by an octave.
Reverse Playback
Once recorded, a signal can be played backwards. This is done by simply reading the
buffer in the opposite order it was written. In this mode of operation, record and play back
are not possible at the same time.
Reverse playback makes the instruments sound different, if not rather strange. The attack,
sustain and decay of the instruments will alter and the spoken word also sound foreign.
Tutorial 3: Record and Playback
14 May, 2017