103 lines
8.1 KiB
Plaintext
103 lines
8.1 KiB
Plaintext
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ST 520/1040 - Stereo Modification
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---------------------------------
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Introduction.
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-------------
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If you have a stereo sound monitor, e.g. the Philips CM 8833, you can
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modify the sound output of the ST to provide a true stereo signal. The sound
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chip, YM2149, has three independent sound channels, A,B and C. In the
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production ST these are simply joined together, hence the monophonic or single
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channel output signal to the monitor (or TV via the modulator in the STM or
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STFM). If you are highly experienced at electronics and are willing to modify
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your ST's circuit board, you might like to attempt the following work. You
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will need to cut tracks on the board. Obviously the internal metal screen has
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to be removed as well. If your machine is still under warranty, this will
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certainly invalidate it. I take no responsibility for any damage you cause to
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your machine. It is entirely your responsibility if you decide to undertake
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the work.
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The circuit.
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------------
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A circuit has to be assembled on a small piece of circuit board. I used
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a short strip of veroboard. The circuit is needed to place the three sound
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channels in different positions in the sound field. Channel A will come out
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on the LEFT, channel B in the centre (i.e. fed to both LEFT and RIGHT equally,
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this is termed panning) and channel C on the RIGHT. The result is a true
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spread of stereo sound between stereo speakers or headphones.
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I have not given a component layout or a list of components since there
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are so few. The total cost should be less than 2 pounds and some of you might
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be able to build it from your spares box. The circuit is given below.
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YM2149 pin 4 >----------------:
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pin 3 >----------------(---: :---------------------------> left
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:-----------(---(--:
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pin 38 >----(-----------(---(--------------O---[ ]-----O--> right
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| | | | 10k |
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O---[ ]----O :----: | |
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| 10k | | | |
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O-----[ ]--(--------O---------(---[ ]-----O
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| 10k | | | 10k |
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[ ]8k2 [ ]1k [ ]1k [ ]1k [ ]8k2
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[ ] [ ] [ ] [ ] [ ]
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pin 1 >----O-----------O--------O---------O------------O-->earth
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[ ] or [ ] ( bridge, no join : bend
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[ ] resistor
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O join
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ST modifications.
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-----------------
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The circuit board around the YM2149 has to be cut carefully to separate
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the three channesl. Pins 3,4 and 38 are joined. 3 and 4 are joined at the pins
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on the top of the circuit board (chip side). Carefully scrape away the join.
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On the underside there is a strip joining pins 3 and 38. Scrape a gap in that
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too. The last one is more difficult. A track leaves pin 38 and goes across
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the board to join to the negative end of capacitor C12 (on the 1040 at least )
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which is the coupling capacitor to the sound output amplifier. It is situated
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near the monitor socket. If you trace its path, you will find it travels
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underneath the YM2149, between the rows of pins (underside of the board). This
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is a convenient place to break the circuit to the mono (normal output).
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Fitting the supplementary board.
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--------------------------------
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The extra circuit should be connected by short flying leads to the pins
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of the YM2149; the appropriate leads to pins 1,3,4 and 38. The stereo
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outputs LEFT, RIGHT and EARTH should be taken by screened leads to the monitor
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output socket. I have broken the tracks at the 13 pin DIN socket so that pin
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1 (the normal mono sound output) becomes the left sound channel and pin 2 (the
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composite video output - not used since a stereo monitor is being used!)
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becomes the right sound channel. Obviously you will have to make up a stereo
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monitor lead too.
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And Finally!
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------------
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The stereo modification will give you glorious stereo on music and sound
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effects, where the programmer has made good use of the YM2149's three
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channels. On some arcade games the effect is stunning! However, you will find
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that sound samples will give very hissy results. This is because some
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samplers use the individual channels to take parts or bites(!) of the sound
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waveform. Therefore the sound channels must be joined or added together i.e.
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you must revert to the channels being joined together! What you need is a
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stereo/mono switch. This is easily done. Wire a two pole two way switch so
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that pin 3 can be joined simultaneously to pins 4 and 38 of the YM2149. With
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the switch one way, the pins are separate (stereo) and the other way the are
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joined together (mono, for sampled sound effects). Programs that use ST
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Replay will need to be played in mono.
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Good luck! Copyright of Derek Green 1988. For use in public domain, NOT TO BE
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SOLD for commercial gain.
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