Smidge Posted December 6, 2021 Share Posted December 6, 2021 I need help understanding how to read the TIA schematics, specifically the schematic for the audio circuitry (sheet 4 here). I'm struggling with some of the notation. My goal is to add audio support to my 2600 emulator, Tamera, by first recreating the audio logic in something like BOOLR so I can figure out how the cryptically named modes of AUDCx work (like wtf does "5-bit poly to 4-bit poly" mean). Question 1: What does the circle over intersecting lines mean? This notation appears all over the place. Here's an example: Someone asked this a few years ago on Reddit and actually got a response from someone claiming to have worked on the 7800 at General Computer Company in the 80s. According to that person, the circled intersection represents a wired AND-gate as described here, with the "resistor to nowhere" being a pull-up resistor (effectively a hard-wired 1 to the wired AND-gate I think). Is it correct that these are just AND-logic? Question 2: What are the dual outputs of the audio control registers? Each bit of the audio control register (AUDCx) feeds into a black box with two outputs: Black boxes such as these appear throughout the TIA schematics whenever a logical unit is replicated, and they are typically accompanied by a separate schematic off to the side. Examples can be found on the same schematic sheet as the audio circuit for the player graphics registers and the audio frequency registers. However there is no such explicit schematic for the audio control register. There is a similar sub-unit for the audio frequency circuit (1 input per bit with two outputs), but it is not clear whether it is the same sub-unit used for the audio control registers. Question 3: What are these, transistors? I'm referring to the two similar looking symbols that bump up and have a line over top. Are they transistors acting as switches here? Why does one have an asterisk? How can I tell if the switch is active low or high? Question 4: Why does this NOR-gate have wings? Do the extra bits that stick out on the sides have any special meaning or is it just to add room for more inputs? Question 5: Are the "5-bit poly" and "4-bit poly" circuits shown explicitly as separate units? The TIA documentation mentions the audio circuit uses a 9-bit shift register which does not appear to be in the schematic. However there is what looks to be a 5-bit polynomial shift register (the five D1 boxes under "AUDIO NOISE GEN.") and another 4-bit polynomial shift register on the bottom right (implemented with SR-latches). (The 5-bit feeds into the 4-bit which I suppose gives 9 bits). Are these two units the "5-bit poly" and "4-bit poly" which are mentioned in the documentation for the AUDCx registers? Quote Link to comment Share on other sites More sharing options...
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