8

®

PCM56

SECOND GENERATION SYSTEMS

One method of avoiding the problems associated with a

higher order analog filter would be to use digital filter

oversampling techniques. Oversampling by a factor of two

would move the sampling frequency (88.2kHz) out to a

point where only a simple low-order phase-linear analog

filter is required after the deglitcher output to remove

unwanted intermodulation products. In a digital compact

disc application, various VLSI chips perform the functions

of error detection/correction, digital filtering, and formatting

of the digital information to provide the clock, latch enable,

and serial input to the PCM56. These VLSI chips are

available from several sources (Sony, Yamaha, Signetics,

etc.) and are specifically optimized for digital audio

applications.

Oversampled circuitry requires a very fast D/A converter

since the sampling frequency is multiplied by a factor of two

or more (for each output channel). A single PCM56 can

provide two-channel oversampling at a 4X rate (176.4kHz/

channel) and still remain well within the settling time

requirements for maintaining specified THD performance.

This would reduce the complexities of the analog filter even

further from that used in 2X oversampling circuitry.

FIGURE 9. A Sample/Hold Amplifier (Deglitcher) is Required at the Digital-to-Analog Output for Both Left and Right Channels.

FIGURE 10. Timing Diagram for the Deglitcher Control Signals.

R

2

2.2k

Ω

A

1

(1)

C

1

680pF

C

B

A

Left Channel

Output to LPF

SW

1

MP7512

(Micro Power)

R

4

2.2k

Ω

A

2

(1)

C

2

680pF

C

B

A

Right Channel

Output to LPF

SW

2

MP7512

(Micro Power)

R

1

2.2k

Ω

R

3

2.2k

Ω

NOTE: (1) 1 OPA101AM or 1/4 OPA404KP or 1 OPA606KP or OPA2604.

PCM56

Serial Data

Clock

Latch Enable

Left Channel

Deglitcher Control

Right Channel

Deglitcher Control

A "low" signal on the deglitcher control closes switch "A",

while a "high" signal closes switch "B".

Latch Enable

Right Channel

Serial Data

44.1kHz

Left Channel

Left Channel

Right Channel

Right Channel

Deglitcher Control

Left Channel

Deglitcher Control

t

t

= 1.5µs DAC Settling Time

ω

The deglitcher control signals by timing control logic. The fast settling time of the PCM56 makes it possible to

minimize the delay between left and right channels to about 4.5µs, which reduces phase error at the higher audio frequencies.