CYRUS AV5 TECHNICAL DESCRIPTION
July 2000
4
Digital to analogue conversion
The digital audio output from the DSP is routed back to the CODEC via data lines from pins 39-41. Word
select pin 42 and bit-clock pin 43 to give 3 stereo signals (5.1 channels). The CODEC contains a six
channel DAC. The DAC converts the six digital channels to analogue and the signals are AC coupled to the
filter stage. The filter stages take out the high frequency over-sampling noise. All filters are then AC coupled
to the volume control stage.
Volume controls and output buffers
The volume control stage uses two digitally controlled triple volume control IC’s. The controls are connected
to the control microprocessor via the SPI bus. The volume controls are used to set both the volume and any
channel balance offsets applied by the user to level match between speakers. The volume controls run off +/-
6VDC, regulated from the +/-12VDC rails by T601/602. The output is then buffered by IC604/5/6.
Mute circuit
The output mute circuit T702/7 is connected to the output and short circuits the audio signals. The mute
circuit is used during power up/down and standby and is controlled by the control microprocessor and a little
hardware based circuit to detect power supply condition T101.
Tape output
The tape output is only active when set to decode stereo. DTS cannot be down mixed to stereo and hence
cannot be recorded via the analogue tape out. The left and right audio outputs from the post DAC analogue
filters are connected to the tape output sockets. When set to stereo the tape output sockets are un-muted
(T701/19/20) allowing recording to take place. In all other operation modes the tape output is muted.
Auto calibration microphone
The AV5 includes a facility to allow the automated set-up of channel balance and speaker distance setting.
When using auto level the DSP produces noise on each channel in turn, the control microprocessor will
adjust the volume automatically to achieve the same reading from the supplied microphone for each speaker
channel. The settings are stored in the microprocessor memory and are applied to all volume control settings.
The distance setting works in a similar way except the noise is burst and the time taken for the noise burst to
reach the speaker allows the microprocessor to calculate the speaker distances. The distance settings are
stored in the microprocessor’s memory and the appropriate delays are set in the DSP.
The microphone uses a 1.5V button cell, type L1154, AG13, LR44, 157, V13GA, RW82 or A76. It is
essential that the battery is removed when the microphone is not being used to prevent the battery from
discharging. The battery may discharge over night if left in the microphone.
The output from the microphone is boosted by IC1006, rectified by IC1007B, and averaged by C1025.
When measuring the speaker level T1002 is switched on connecting C1026 to increase the averaging time
constant. The output from the averaging circuit is buffered and sent to an analogue input on the control
microprocessor to analyse level or timing.