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The PSG is quite primitive. It is able to output a square wave and/or white noise in three separate sound channels (named Channel A, B and C). Each channel can be used to output tones and/or white noise.

== PSG Part numbers Some other 8-bit systems used in more sophisticated soundchips such as the CPC during its lifetime ==[https://youtu.be/7pONRbIHT_w C64 SID], the [https://youtu.be/BANwL2sQ0DM NES APU] and the [https://youtu.be/-mdjiWBYIqU Gameboy soundchip].

* GI AY-3-8912 [https:== I//www.cpcwiki.eu/imgs/c/cc/CPC464_PCB_Top_%28Z70378_MC0046A%29.jpg Source]* GI AY-3-8912A [https://www.cpcwiki.eu/imgs/f/f1/CPC6128_PCB_Top_%28Z70290_MC0020B%29.jpg Source]* Microchip AY-3-8912 [https://www.cpcwiki.eu/imgs/c/cf/AmstradCPC464_Z70375_MC0044D_GA40010_PCB_Top.jpg Source]* Microchip AY38912/P [https://www.cpcwiki.eu/imgs/5/5e/CPC464Plus_MC0122B_2700-016P-3_PCB_Top.jpg Source]O Access ==

<br> == The PSG Registers ==has 16 read/writeable data registers, and a write-only index register.

The PSG has While there are only 16 read/writeable data registers, and a write-only the index registeris 8-bit, not 4-bit. Only the The higher 4 least significant -bits of the selected index register number are consideredused as chip select.

{| class="wikitable"|-! Binary !! Hex !! Shape!! Comment |-| 00XX || 00h-03h || <code>\_________ (</code> || same as 09h) |-| 01XX || 04h-07h || <code>/_________ (</code> || same as 0Fh) |-| 1000 || 08h || <code>\\\\\\\\\\</code> || |-| 1001 || 09h || <code>\_________ (</code> || volume remains quiet) |-| 1010 || 0Ah || <code>\/\/\/\/\/</code> || |-| 1011 || 0Bh || <code>\""""""""" (¯¯¯¯¯¯¯¯¯</code> || volume remains high) |-| 1100 || 0Ch || <code>//////////</code> || |-| 1101 || 0Dh || <code>/""""""""" (¯¯¯¯¯¯¯¯¯</code> || volume remains high) |-| 1110 || 0Eh || <code>/\/\/\/\/\</code> || |- | 1111 || 0Fh || <code>/_________ (</code> || volume remains quiet) We can observe that there are only 8 resulting shapes even though they are selected by using a 4-bit value.|}

Writing to this register ==== Bit 2 (re-Attack)starts ==== This bit is only responsible for the starting point of the envelopeand the direction.We have: volume = 0 if attack else 15 # Start at 0 for attack, 15 for decay direction = 1 if attack else -1 # Upward for attack, downward for decay ==== Bit 3 (Continue), Bit 1 (Alternate) and Bit 0 (Hold) ==== When bit3 = 0, volume and direction at the end of a period are always 0. When bit3 = 1, bit1 and bit0 determine what happens to volume and direction at the end of a period. ==== Algorithm ====  def envelope_step(volume, direction, shape): volume += direction if volume > 15 or volume < 0: match shape: case 8 | 12: volume &= 0x0f # direction is unchanged case 10 | 14: direction *= -1 volume += direction case 11 | 13: direction = 0 volume = 15 case _: direction = 0 volume = 0 return volume, direction

This register receives data from the CPC keyboard (or joystick), for more information read the chapter about the [[Programming:Keyboard_scanning|CPC Keyboard Matrix]].  This register can be also used as output port by setting bit 6 of the PSG control register to 1 (that would allow to use the six data pins of the joystick connector to output data to external hardware).

 The [[Aleste 520EX]] (russian CPC clone) is a special case: does have it has a 8910 Yamaha YM2149F chip, with PSG SSG Port B being used as 8bit printer port data.

The noise generator uses a [https://en.wikipedia.org/wiki/Linear-feedback_shift_register Linear-Feedback Shift Register] algorithm. The random number generator of the 8910 is a 17-bit shift register.  According to [https://github.com/mamedev/mame/blob/master/src/devices/sound/ay8910.h MAME]: The input to the shift register is bit0 XOR bit3. Bit0 is the output. This was verified on AY-3-8910 and YM2149 chips. However, the algorithm is described in detail in the [[Media:Microchip ay8930.pdf|AY-8930 datasheet]]. And it disagrees with MAME, the input to the shift register is bit0 XOR bit2. And it seems bit1 is the output. [[File:AY38910A noise block diagram.png]]

The PSG chip family is composed of 3 variants: the AY-3-8910 with two 8-bit I/O ports, the AY-3-8912 with one 8-bit I/O port and the AY-3-8913 with no I/O port. In addition to the CPC, these chips were also === IC models used in the [[ZX Spectrum]], [[MSX]], [[Oric]], [[Vectrex]], [[Intellivision]] and in the Mockingboard expansion for the [[Apple II]].CPC ===

The PSG chip competed with These are the DCSG (Digital Complex Sound Generator) chip family (SN76489, SN94624, TMS9919) ones known to be used in the CPC by Texas Instrumentslooking at pictures of CPC mainboards. The DCSG has similar sounding features except that it does not have any envelope control and that its noise generator has its own dedicated channelAll should operate almost identically.

Toshiba T7766A, Winbond WF19054, JFC 95101 and File KC89C72 are clones of the PSG* GI AY-3-8912 [https://www.cpcwiki.eu/imgs/c/cc/CPC464_PCB_Top_%28Z70378_MC0046A%29. jpg Source]* GI AY-3-8912A [https://wikiwww.agiricpcwiki.ninjaeu/sound_chip_clonesimgs/f/f1/CPC6128_PCB_Top_%28Z70290_MC0020B%29.jpg Source]* Microchip AY-3-8912 [https:index //www.cpcwiki.eu/imgs/c/cf/AmstradCPC464_Z70375_MC0044D_GA40010_PCB_Top.jpg Source]* Microchip AY38912/P [https://www.cpcwiki.eu/imgs/5/5e/CPC464Plus_MC0122B_2700-016P-3_PCB_Top.jpg Source]

The PSG chip family is composed of 3 variants:* the AY-3-8910, with two 8-bit I/O ports and a 40-pin package* the AY-3-8912, with one 8-bit I/O port and a 28-pin package* the AY-3-8913, with no I/O port and a 24-pin packageIn addition to the CPC, these chips were also used in the [[KC Compact]], [[ZX Spectrum]], [[MSX]], [[Oric-1/Atmos|Oric]], [[EG2000 Colour Genie]], [[Vectrex]], [[Intellivision]] and in the Mockingboard expansion for the [[Apple II]]. There are also PSG clones: Toshiba T7766A, Winbond WF19054, JFC 95101 and File KC89C72. [https://wiki.agiri.ninja/sound_chip_clones:index Source] Yamaha produced the SSG (Software-controlled Sound Generator) chip family (YM2149F, YM3439, YMZ294, YMZ284, YMZ285) which is a quasi-clone of the PSG. The main difference is that the envelope counter on the PSG has 16 steps. On the SSG it has twice the steps, happening twice as fast. This chip equips the [[Atari ST]], [[Aleste 520EX]] and [https://www.msx.org/wiki/Yamaha_YM2149 some MSX computers]. It is also used in the [[PlayCity]] expansion. The SSG has also been integrated as a component inside some of the arcade soundchips of the Yamaha OPN family. That's why you can find it in the [[Neo-Geo]] as a component inside the YM2610 soundchip. And the same is true for the OPN3 soundchip of the [[Play2CPC]] expansion. The chip is clocked differently depending on the computer: ZX Spectrum: 1773400 Hz ; Pentagon: 1750000 Hz ; MSX: 1789772 Hz ; CPC: 1000000 Hz ; Oric: 1000000 Hz ; Atari ST: 2000000 Hz.

The PSG chip competed with the DCSG (Digital Complex Sound Generator) chip family (SN76489, SN94624, TMS9919) by Texas Instruments. The DCSG has similar sounding features except that it does not have any envelope control and that its noise generator has its own dedicated channel. == Datasheet = Replacing the AY-3-8912 in the CPC with an AY-3-8910(A) ===* [[Media:Ay3In case that the AY-3-8912 needs to be replaced in the CPC it can become a hurdle these days to get a real AY-3-891x8912 especially for an acceptable price.pdf| Instead of the original AY-3-891x datasheet8912 it's possible to use an AY-3-8910(A) instead with the help of a small adapter board. An open source adapter board can be found here: https://github.com/etomuc/CPC-AY-3-8910-to-8912-adapter For tips regarding the desoldering of ICs see this Wiki page: [[IC Repair]] <br>

[[Category:Music and sound| ]][[Category:Video contents]][[Category:CPC Internal Components]]