Difference between revisions of "PSG Registers R11 - R13"

From Intellivision Wiki
Jump to: navigation, search
m (Protected "PSG Registers R11 - R13" ([edit=autoconfirmed] (indefinite) [move=autoconfirmed] (indefinite)))
(No difference)

Latest revision as of 09:03, 4 December 2010

Volume levels and envelope selects for channels A, B and C. Channel A uses R11, channel B uses R12 and channel C uses R13. See PSG Channels and PSG Envelope Generator for more details.

       ee          selects the envelope magnitude
                     00  No envelope selected.  PSG uses the flat volume in 'vvvv' instead.
                     01  Use envelope volume shifted right by 2
                     10  Use envelope volume shifted right by 1 
                     11  Use envelope volume as-is
       vvvv        indicates the flat volume to use if no envelope selected (ee is 00).

Some variants of the AY-3-891x family do not implement both envelope select bits. Those devices only allow selecting the unmodified envelope or a flat volume. On those devices, bit 5 always reads as zero (0). To ensure compatibility across both variants of the PSG, programs should always write ones (1) to both 'e' bits, and also should not rely on a one (1) written to bit 5 reading back as one (1) or zero (0). (Writing zero (0) to bit 5 and subsequently reading it always gives zero (0).)