Electronics > Projects, Designs, and Technical Stuff
How were color bars broadcast on TV?
Ben321:
I know that some stations would broadcast the SMPTE color bars or other similar color bar patterns when they weren't broadcasting content. I also know there's 2 types of color bar patters. 75% intensity and 100% intensity. With 75% intensity, the maximum excursion of the chroma carrier for the yellow bar would be 100IRE, but the white bar would actually be approximately 77IRE (75*0.95 +7.5=76.875). The other standard color bars are the 100% intensity colors, and have the white bar at 100IRE, but the maximum excursion for the chroma carrier for the yellow bar would be at approximately 131 IRE. However I think this could pose a problem with transmitters. The current NTSC standard has 120IRE being considered full modulation, and because transmitters use inverse AM modulation, at full modulation the carrier signal completely stops transmitting. And above full modulation, the carrier wave's phase is actually inverted. So if you put 131 IRE signal level for a full intensity yellow bar into a transmitter that has full modulation at only 120 IRE, that would cause a problem.
When there's a phase inversion of a signal presented to the AM demodulator in the TV receiver, this will result in various artifacts in the demodulated signal. How is this situation resolved when a studio transmits a full intensity color bars pattern? Or do they simply never transmit a color bar pattern that's above 75% intensity for over-the-air transmission of the color bars, and use the 100% intensity color bars pattern only for calibrating equipment directly connected by wire in the studio?
mansaxel:
The 75% bars are the only ones used in production and transmission as far as I can tell. At least here in what-used-to-be-PAL-land this now is 98% archaeology since the production chain since 20 years is SDI (or encoded with compression to transmit over IP, to be decoded back to SDI later) and transmission is DVB-T / DVB-T2.
These days transmission is tested using Wireshark for protocols like SMPTE ST2110. Yes, our instrument has bars, but the most work is done using the timing and packet stream analysis tools.
"All broadcast engineers are closet time-nuts"
(Another forum member to me, in privmsg)
Ben321:
--- Quote from: mansaxel on October 28, 2021, 06:14:27 am ---The 75% bars are the only ones used in production and transmission as far as I can tell.
--- End quote ---
By production do you mean making video intended to be transmitted? Or do you mean calibrating studio monitors?
--- Quote from: mansaxel on October 28, 2021, 06:14:27 am ---At least here in what-used-to-be-PAL-land this now is 98% archaeology since the production chain since 20 years is SDI (or encoded with compression to transmit over IP, to be decoded back to SDI later) and transmission is DVB-T / DVB-T2.
--- End quote ---
I don't know much about European technology, as I live in the US, and my question was about NTSC (even the spec about 120IRE being full moduluation, is an NTSC standard, so I don't think info about the PAL system will help me much here).
--- Quote from: mansaxel on October 28, 2021, 06:14:27 am ---These days transmission is tested using Wireshark for protocols like SMPTE ST2110. Yes, our instrument has bars, but the most work is done using the timing and packet stream analysis tools.
"All broadcast engineers are closet time-nuts"
(Another forum member to me, in privmsg)
--- End quote ---
Wireshark really? I thought that was a network protocol analyzer for internet and LAN type networks (like 802.11 wireless, and Ethernet wired networks). I didn't know it could analyzer protocols that weren't designed specifically for computer networking.
Also I'm surprised you even need bars in digital video. Digital video makes sure that the output is a bit-for-bit accurate reproduction of the input, so no possibility to have wrong phase of the chroma carrier or other issues that would make the color wrong output in analog video. Digital video means that all colors are precisely accurate.
vk6zgo:
--- Quote from: Ben321 on October 28, 2021, 05:17:35 am ---I know that some stations would broadcast the SMPTE color bars or other similar color bar patterns when they weren't broadcasting content. I also know there's 2 types of color bar patters. 75% intensity and 100% intensity. With 75% intensity, the maximum excursion of the chroma carrier for the yellow bar would be 100IRE, but the white bar would actually be approximately 77IRE (75*0.95 +7.5=76.875). The other standard color bars are the 100% intensity colors, and have the white bar at 100IRE, but the maximum excursion for the chroma carrier for the yellow bar would be at approximately 131 IRE. However I think this could pose a problem with transmitters. The current NTSC standard has 120IRE being considered full modulation, and because transmitters use inverse AM modulation, at full modulation the carrier signal completely stops transmitting. And above full modulation, the carrier wave's phase is actually inverted. So if you put 131 IRE signal level for a full intensity yellow bar into a transmitter that has full modulation at only 120 IRE, that would cause a problem.
When there's a phase inversion of a signal presented to the AM demodulator in the TV receiver, this will result in various artifacts in the demodulated signal. How is this situation resolved when a studio transmits a full intensity color bars pattern? Or do they simply never transmit a color bar pattern that's above 75% intensity for over-the-air transmission of the color bars, and use the 100% intensity color bars pattern only for calibrating equipment directly connected by wire in the studio?
--- End quote ---
Analog TV transmitters do not "cut carrier" under any circumstances.
Even if you did hit it with excessive video modulation levels, the inbuilt circuitry would clip the signal before that happened.
mansaxel:
--- Quote from: Ben321 on October 28, 2021, 07:44:45 am ---
--- Quote from: mansaxel on October 28, 2021, 06:14:27 am ---The 75% bars are the only ones used in production and transmission as far as I can tell.
--- End quote ---
By production do you mean making video intended to be transmitted? Or do you mean calibrating studio monitors?
--- End quote ---
Production of TV.
--- Quote from: Ben321 on October 28, 2021, 07:44:45 am ---
--- Quote from: mansaxel on October 28, 2021, 06:14:27 am ---At least here in what-used-to-be-PAL-land this now is 98% archaeology since the production chain since 20 years is SDI (or encoded with compression to transmit over IP, to be decoded back to SDI later) and transmission is DVB-T / DVB-T2.
--- End quote ---
I don't know much about European technology, as I live in the US, and my question was about NTSC (even the spec about 120IRE being full moduluation, is an NTSC standard, so I don't think info about the PAL system will help me much here).
--- End quote ---
As I wrote, this must be only of historical interest. It's SDI (SD, HD or 3G) all the way unless it's gone ST2110 or NDI. Signal is analog only before the CCD in the camera. The rest is bits and bytes, until it's decoded and displayed in people's TVs. Except for the poor sods who have analog cable.
We currently did some channel and encoding reshuffles here, turning off a lot of the DVB-T net and moving stuff to DVB-T2 instead. Viewers on small cable nets who relied on central decoding into analog channels were hit in a lot of places, because these back-asswards systems were unable to cope.
--- Quote from: Ben321 on October 28, 2021, 07:44:45 am ---
--- Quote from: mansaxel on October 28, 2021, 06:14:27 am ---These days transmission is tested using Wireshark for protocols like SMPTE ST2110. Yes, our instrument has bars, but the most work is done using the timing and packet stream analysis tools.
"All broadcast engineers are closet time-nuts"
(Another forum member to me, in privmsg)
--- End quote ---
Wireshark really? I thought that was a network protocol analyzer for internet and LAN type networks (like 802.11 wireless, and Ethernet wired networks). I didn't know it could analyzer protocols that weren't designed specifically for computer networking.
--- End quote ---
Modern TV production networks are just that, networks. Video is encoded into multicast RTP streams in the camera controller and consumers just siphon off the multicast. Mind you, this does take some network; a 1080i stream is ~1500Mbits/s. Wireshark can drink from that packet hydrant just fine, but there's some processing and sorting required to make sense of it.
--- Quote from: Ben321 on October 28, 2021, 07:44:45 am ---Also I'm surprised you even need bars in digital video. Digital video makes sure that the output is a bit-for-bit accurate reproduction of the input, so no possibility to have wrong phase of the chroma carrier or other issues that would make the color wrong output in analog video. Digital video means that all colors are precisely accurate.
--- End quote ---
Yes, the colours reliably are as weird as they were in the studio. As I entered the field of broadcast engineering 23 years ago, the entry prize was a Spectrol trimmer to put in your pocket protector, for EQing distribution amps. These days, it's a laptop with a web browser and a SSH client.
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