Audio Engineering

512 Chapter 16

LSB, both the initial and the long-term precision of the electronic components used to
defi ne the size of the MSB would need to be better than 0.00305%. (A similar need for
accuracy obviously also exists in the ADC used in recording.)

Bearing in mind that even a 0.1% tolerance component is an expensive item, such an
accuracy requirement would clearly present enormous manufacturing diffi culties. In
addition, any errors in the sizes of the steps between the LSB and the MSB would lead
to waveform distortion during the encoding/decoding process: a distortion that would
worsen as the signal became smaller.

Individual manufacturers have their own preferences in the choice of digital-to-analogue
conversion (DAC) designs, but a Philips system is illustrated, schematically, by way
of example, in Figure 16.10 , is an arrangement called “ dynamic element matching. ” In
this circuit, outputs from a group of current sources, in a binary size sequence from 1
to 1/128, are summed by the amplifi er A1, whose output is taken to a simple “ sample
and hold ” arrangement to recover the analogue envelope shape from the impulse stream
generated by the operation of the A1 input switches (S 1 –S 8 ). The required precision of
the ratios between the input current sources is achieved by the use of switched resistor–
capacitor current dividers, each of which is only required to divide its input current into
two equal streams.





Output

Gain set resistor

A1

Clock

Sample and

hold

0V

0V

MSB LSB

Current sources

Diode/transistor

switches S 1 S 2 S 3 S 4 S 5 S 6 S 7 S 8

1 1/2 1/4 1/8 1/16 1/32 1/64 1/28

2

1

1/2

1/4

1/8

etc.

2

2

2

2

Figure 16.10 : Dynamic matching DAC.