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 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 7 January, 2002

---