Frequency reference from television set? - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: March 11, 2019, 06:50:05 am
content: Back in the days of analog tv some stations broadcast a very accurate line sync signal. The ABC in Australia for example used a rubidium frequency standard to derive 15.625kHz sync pulses. You could pick the horizontal sweep frequency off your tv with a loose piece of wire draped over the back of the set, multiply it x64 with a 4046 PLL and hey presto a very accurate 1MHz to play with. Now in this modern age of digital TVs and suchforth is there a similarly accurate and easy to find signal inside the tv that could also be used as a poor man’s frequency reference? Let’s ignore GPSDOs please.

author: DaJMasta
author_rank: Super Contributor
timestamp: March 11, 2019, 07:13:49 am
content: You could still use broadcast frame data, but power line cycles are a pretty good choice and can be very easy to sense/amplify as long as you can deal with the lower frequency. You can also use cellular time, since every tower has pretty precise timekeeping if you have a radio to interface with it. You've also got old fashioned radio time signals, though at least in the US, some of the older standards have been/are being phased out.I'd assume any AM commercial broadcast would be good too - their frequency is going to be tightly disciplined and spot on, and since there isn't any frequency variation component to the modulation, it should be easy to extract the carrier.You could extract frame timing info in a similar way from TVs nowadays, but it's going to be the local refresh timing based off the internal oscillator, I suspect.

author: IconicPCB
author_rank: Super Contributor
timestamp: March 11, 2019, 08:16:14 am
content: Back then... CH9 used to be on air 24hours a day...The frequency ref at HP labs in Melbourne used to use ch9 as a reference to check own standard.

author: David Hess
author_rank: Super Contributor
timestamp: March 11, 2019, 08:18:13 am
content: The only TV stations where this was generally the case were network stations when they were retransmitting network material. Now that frame buffering is trivial, there is no need to phase lock all stations.AM and FM broadcast station carriers are not particularly accurate that I have noticed but some might be phase locked to a GPSDO now just because it is inexpensive to do.The carrier from the various time broadcast standards can be used with a direct tuned receiver (1) but if you rely on sky-wave propagation, the Doppler shift from the changing height of the ionosphere ruins the accuracy. I have watched this before on an oscilloscope which is kind of eerie.(1) I've seen direct tuned designs which did this but couldn't you downconvert to an IF, filter, and then upconvert with the same local oscillator to recover the original signal without any LO error?

author: richard.cs
author_rank: Super Contributor
timestamp: March 11, 2019, 05:42:58 pm
content: BBC Radio 4 longwave has an accurately controlled carrier suitable for use as a frequency standard, but it does have phase modulation on it. With a suitable (read: big) antenna it is receivable worldwide. The future is a bit uncertain though, the BBC have announced they will let it die when they run out of spare transmitting valves, although that is a very poor argument from a technical perspective.

author: ocw
author_rank: Regular Contributor
timestamp: March 11, 2019, 06:20:34 pm
content: Many, but far from all stateside DTV stations have their pilot frequencies frequencies linked to the GPS satellites. That frequency is 309,440.559 Hz above the lower edge of their 6 MHz bandwidth. Around 30 - 40% of the local DTV stations have GPS linked frequencies.

author: Benta
author_rank: Super Contributor
timestamp: March 11, 2019, 08:22:06 pm
content: What's really accurate is the colour burst - or rather the TV set's local colour oscillator, which is synced to it. If you can tap into that, you'll have a nice reference. Downside is, the frequency is chosen for minimum interference with line and frame, so it's not easy to synthesise a nice frequency from it.