# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1330820548 -3600
# Node ID 316b8e54c28b1c2ea63b3b98af281fb4ba771750
# Parent  4e5f93f2cae7ecaf2745b0791ca1ea07e929a0b4
Added video, audio and general notes.

diff -r 4e5f93f2cae7 -r 316b8e54c28b ULA.txt
--- a/ULA.txt	Sun Feb 26 23:22:17 2012 +0100
+++ b/ULA.txt	Sun Mar 04 01:22:28 2012 +0100
@@ -1,3 +1,24 @@
+Principal Design and Feature Constraints
+----------------------------------------
+
+The features of the ULA are limited by the amount of time and resources that
+can be allocated to each activity necessary to support such features given the
+fundamental obligations of the unit. Maintaining a screen display based on the
+contents of RAM itself requires the ULA to have exclusive access to such
+hardware resources for a significant period of time. Whilst other elements of
+the ULA can in principle run in parallel with this activity, they cannot also
+access the RAM. Consequently, other features that might use the RAM must
+accept a reduced allocation of that resource in comparison to a hypothetical
+architecture where concurrent RAM access is possible.
+
+Thus, the principal constraint for many features is bandwidth. The duration of
+access to hardware resources is one aspect of this; the rate at which such
+resources can be accessed is another. For example, the RAM is not fast enough
+to support access more frequently than one byte per 2MHz cycle, and for screen
+modes involving 80 bytes of screen data per scanline, there are no free cycles
+for anything other than the production of pixel output during the active
+scanline periods.
+
 Timing
 ------
 
@@ -269,7 +290,11 @@
 those outputs are strictly digital and can only be set to a "high" and "low"
 value, then only the existing eight colours are possible. If a modern ULA were
 able to output analogue values, it would still need to be assessed whether the
-circuitry could successfully handle and propagate such values.
+circuitry could successfully handle and propagate such values. Various sources
+indicate that only "TTL levels" are supported by the RGB output circuit, and
+since there are 74LS08 AND logic gates involved in the RGB component outputs
+from the ULA, it is likely that the ULA is expected to provide only "high" or
+"low" values.
 
 Palette Definition Lists
 ------------------------
@@ -467,6 +492,16 @@
 especially since accessing regions of the screen would involve incrementing
 pointers by amounts that are inconvenient on an 8-bit CPU.
 
+Genlock Support
+---------------
+
+The ULA generates a video signal in conjunction with circuitry producing the
+output features necessary for the correct display of the screen image.
+However, it appears that the ULA drives the video synchronisation mechanism
+instead of reacting to an existing signal. Genlock support might be possible
+if the ULA were made to be responsive to such external signals, resetting its
+address generators upon receiving synchronisation events.
+
 Enhanced Sound
 --------------
 
@@ -487,6 +522,17 @@
 As with a hardware sprite function, waveforms could be uploaded or referenced
 using locations as registers referencing memory regions.
 
+Sound Input/Output
+------------------
+
+Since the ULA already controls audio input/output for cassette-based data, it
+would have been interesting to entertain the idea of sampling and output of
+sounds through the cassette interface. However, a significant amount of
+circuitry is employed to process the input signal for use by the ULA and to
+process the output signal for recording.
+
+See: http://bbc.nvg.org/doc/A%20Hardware%20Guide%20for%20the%20BBC%20Microcomputer/bbc_hw_03.htm#3.11
+
 BBC ULA Compatibility
 ---------------------
 
@@ -576,3 +622,8 @@
   CAS MO (motor relay output)
 
   ÷13 IN (~1200 baud clock input)
+
+References
+----------
+
+See: http://bbc.nvg.org/doc/A%20Hardware%20Guide%20for%20the%20BBC%20Microcomputer/bbc_hw.htm