Adding Audio to the Bush STB

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Adding Audio

Here are some more details about getting the sound working on the Bush Set Top Internet boxes. AFAIK the PWB artwork for the sound circuitry is present for the IBX100/101/250/260. Sound circuitry is not present on the IBX200/201/202 which use a different ('cost reduced'?) PWB ??

None of the Bush Boxes have the sound components actually fitted.

Some of the following is probably incorrect. Use with care and check all information.

Discussion

The ARM7500FE chip has the same 16-bit sound interface that is used on the VIDC2 found in the RiscPC. Relevent pins of the ARM7500FE for the 16-bit sound are

68	SDO	16-bit serial sound stream
69	SCLK	Sound Clock In - requires a 11.2896MHz xtal oscillator.
70	SDCLK	Sound Clock Out - (i.e. from the ARM7500FE)
71	WS	Word Select - selects right or left channel

RiscPC Sound Cards

Early RiscPCs (RPC600s?) used the original Archimedes 8-bit sound system. Acorn fitted a six-pin header to the motherboard. This header was intended to be used for future expansion cards for full VIDC2 16-bit sound. I suspect that the Philips 16-bit D-> was not ready in time for the RPC ??

16-bit RiscPC sound cards were made by at least 2 different companies. The sound card I fitted to my IBX100 was made by ESP for Acorn.

RPC sound cards contain

11.2896 MHz	crystal oscillator.
Philips TDA1545A	serial 16-bit Digital->Analogue Converter (DAC)
Philips TDA1308	line driver (op amp?)
74HCT74D	My other RPC sound card uses a different IC here.
	Plus some capacitors and resistors
	Output capacitors are 100uF 16v

Wiring RPC Sound Card To Bush Box

I used an IBX100. Soldered a two pin header to the three pins intended for the sound output on the PWB. Centre pin is ground. Can use this ground pin to supply 0v to the 16-bit sound card.

6-pin header of the sound card has following pin designations

1	Ground 0v
2	SCLK
3	WS
4	SDCLK
5	SData
6	+5v

These pins need to connect to the IBX motherboard.

1	Ground	centre pin of 3-pin header on sound output of motherboard
2	SCLK	R20 pin nearest the ARM7500FE
3	WS	IC2 pin 2
4	SDCLK	IC1 pin 9
5	SData	IC1 pin 13
6	+5v	IC1 pin 14 (or elsewhere)

Fitting Sound Components to the IBX Motherboard.

Originally I was going to remove the components from the RPC soundcard and fit them into the relevent positions on the IBX motherboard.

Components need to be

X2	Crystal Oscillator 11.2896 MHz
(X3	This is the HCLK Xtal for VGA monitor timing and not for sound)
IC1	Some sort of 74HCxx TTL - not sure of type
IC2	Philips TDA1545A DAC (Crickewood Electronics sell an equivilent)
IC3	Line Driver TDA1308 or similar
C6, C13	Ouput Capacitors 100uF 16 v ?
PL1A	3-pin header for sound output (line/headphone levels)
	Plus some other resistors and capacitors...

Sound Modules

Sound Modules are not part of RiscOS-NC. Need to soft load the sound modules using an old-style Iomega 100MByte ZIP drive. I extracted the sound modules from my RiscPC using the excellent !ZAP editor (where are similar freeware programmes of this quality for EPOC32?).

Add these lines to your boot file on the ZIP disk ($.auto)
*rmload izipfs:$.modules.sounddma
*rmload izipfs:$.modules.soundchann
*rmload izipfs:$.modules.soundsched
*rmload izipfs:$.modules.wavesynth
*rmload izipfs:$.modules.stringlib

Assumes that you have a directory 'modules' to keep the modules in. The old wavesynth and stringlib are only required for the old style sounds, like the 'beep' on reset etc.

I use the amplayerfp module to play MP3s. At 128-bit stereo 44.1 KHz there's not much processing power remaining to do much else on the Bush Box ! That said hats-off to the team that wrote the amplayer sound modules.

Not tried using Bush !Fresco with sound yet.

Ralph

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