How does this odd Composite Sync + Lum mixer work? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: October 13, 2023, 08:37:05 am
content: Why do they have the diode there? If the CSync signal is high, you could just make that part of the mixer values. So you wouldnt need the 1.6k pull up;instead you remove CR4, and replace R51 with a 2.7k instead of 750 ohm. Remove R46, and change R52 from 2k to 1.6k.Or no? is there something funky with the composite sync that needs to have that diode there?I've attached the internal block diagram of the GTIA if you wanna take a peek. Comp sync output circuit at the top of page 3.

author: PetefromCanada
author_rank: Newbie
timestamp: October 13, 2023, 01:27:15 pm
content: its fairly typical design choice, but perhaps a bit more interesting than one would expectThere is a specification, RS170A, which defines the relationship between composite sync along and the rest of the video signal. Looking at the attached waveform, notice the amplitude range of sync must be from 0 to -30 while the video portion, which the user will see on the screen, must be between 0 and 100. its the ratio between these which matters to the monitor. This circuity establishes that the video signal is RS170a compliant and as such is expected to work with any video monitorThe decoding circuitry would assume this ratio so that it could effectivly "clamp" this signal to DC so as to use a simple diode arrangement to seperate the syncronizing component from the active video component. The idea being that the circuitry in the consumers' television would be in expensive..you can trace down the specification by reading up on RS170 where NTSC is defined or its earlier B/w version IEC. if you have an older scope you may see a TV setting in the triggering function which will use the clamping with an approriate filter to trigger the scope at either horizontal rate @~ 15Khz or vertical @~ 60 Hz

author: PetefromCanada
author_rank: Newbie
timestamp: October 13, 2023, 02:38:24 pm
content: its fairly typical design choice, but perhaps a bit more interesting than one would expectThere is a specification, RS170A, which defines the relationship between composite sync along and the rest of the video signal. Looking at the attached waveform, notice the amplitude range of sync must be from 0 to -30 while the video portion, which the user will see on the screen, must be between 0 and 100. its the ratio between these which matters to the monitor. This circuity establishes that the video signal is RS170a compliant and as such is expected to work with any video monitorThe decoding circuitry would assume this ratio so that it could effectivly "clamp" this signal to DC so as to use a simple diode arrangement to seperate the syncronizing component from the active video component. The idea being that the circuitry in the consumers' television would be in expensive..you can trace down the specification by reading up on RS170 where NTSC is defined or its earlier B/w version IEC. if you have an older scope you may see a TV setting in the triggering function which will use the clamping with an approriate filter to trigger the scope at either horizontal rate @~ 15Khz or vertical @~ 60 Hz

author: PetefromCanada
author_rank: Newbie
timestamp: October 13, 2023, 06:31:22 pm
content: With respect to the schematic you posted, it is indeed unwise to have the digital lumance value at anything other than zero as ulimatly there is a summation of LUMO to LUM4 + CSync happening. If I follow the circuit, CSync is brought low for horizontal and vertical sync and high during active , ie non blanked, video. IN short the amplitde of the sync portion of the signal would become a variable which is "bad". And bad for a great host of reasons. Firstly asuming the video is modulated at a later stage the amplitude of the sync pulse could be driving an automated gain control in the RF section. secondly As video is typically transmitted as a radio signal it can't have a DC component. accordingly a DC refernce will be derived interally from the sync circuit as part of a analogue clamping circuit sometime referred to as DC restoration. effectivly sets the "black level". with out this the tv becomes unwatchable as the average brightness will jump around.The chip may well let you output non zero during vertical blanking as the closed caption data is transmitted during this blanking period and as well a few lines of video could contain test pattern information during vertical blanking,I hope thats not too much information. NTSC is one very complicated video signal!

author: wasedadoc
author_rank: Super Contributor
timestamp: October 13, 2023, 07:22:04 pm
content: Quote from: PetefromCanada on October 13, 2023, 02:38:24 pm1. Incorrect. Should be 0 and -40 not 0 and -30 for NTSC. 140 IRE units is 1 Volt video. PAL uses the 3:7 ratio.https://en.m.wikipedia.org/wiki/IRE_(unit)2. No waveform was attached.

author: PetefromCanada
author_rank: Newbie
timestamp: October 13, 2023, 07:34:49 pm
content: thank you for correcting a typo.waveform is clearly attached in an earlier reply