da-omvandlare med omklockning och uppsampling

With this project we try to accomplish an Audio Digital-to-Analogue Converter, DAC, with as low jitter as possible. Jitter is by definition when a system’s clock signal deviates from the ideal case. The effect of jitter is that it introduces noise and distortion into the signal. A pure tone of one single frequency will be modulated by the timing jitter which causes a “spreading” of the signal frequency.

Jitter can be both random and correlated, the latter being the worst and can produce rather bizarre modulation effects. Random jitter originates mainly from the signal source and the transmission between systems. Low cost crystal oscillators in the signal source inhibit certain amounts of jitter which is uncorrelated with the timing errors in the DAC. This is removed by ‘reclocking’ the signal with a more stable crystal oscillator and a Phase Locked Loop, PLL.

Unregulated power supplies are the major cause of correlated jitter. When a digital component draws current from the supply, the voltage takes a dive which in turn affects other digital components on the same supply. This is removed by using multiple low noise stabilized supplies.

In this project we build a complete Digital-to-Analogue converter using several digital components from various manufacturers. We receive a digital signal with a Digital Audio Receiver, which converts the signal into serial data and clock.

These signals are then reclocked using standard digital logic and a very accurate crystal oscillator. A sample-rate-converter converts the signal to 96KHz, 24bits.

Finally we use an audio converter convert these signals to analogue.

To control our DAC we have designed a computer core using a standard processor from Motorola, 32Kb of SRAM and EPROM and a programmable logic array from Lattice Semiconductors.

The result is a working DA-converter with surprisingly good sound quality. We have learned a great deal and highly recommend this course.

Henrik Johansson, e95hjo@efd.lth.se

Susanne Dunér, e95sdu@efd.lth.se