Analog TV reception in a spacecraft moving relative to the Earth

[Dean Talboys]

TL;DR Summary

How fast would you need to be travelling for the Doppler effect to prevent reception?

During the Apollo 11 mission to the Moon there at least two occasions when the crew indicate they are watching live TV broadcasts from Earth. We know they originate from Earth because they are live TV transmissions that were received by network TV and recorded on video that is available in the NASA archives. We also know from essays by NASA engineers how a sub-standard TV signal was generated aboard the Apollo command module, how it was received on Earth, and how it was processed to produce a standard NTSC signal suitable for broadcast on commercial TV networks. Part of that process involved synchronization of the signal received from Apollo 11, which was travelling at around 3,000 mph away from or towards the Earth, the idea being that the Doppler effect would prevent the signal from being displayed on analog TV where the screen refresh is important for a stable picture. That process was resolved using two VTRs sharing one magnetic tape (although I have not been able to find a graphic explaining the setup).

My question is, given the light speed of EM waves, would the high speed of the spacecraft really have any impact on synchronization of the signal? If not, it is quite reasonable to assume the crew were able to watch live TV from Earth. However, that would beg the question why NASA introduced the two VTRs in the first place. There again if it was necessary, how were the crew able to watch TV without a similar setup aboard the spacecraft?

Thanks in advance.

 

[Ibix]

The situation is symmetric - if I have trouble receiving a signal from you due to your speed relative to me, you will have the same trouble with a signal from me.

The craft was doing around one two hundred thousandth of light speed, so the Doppler shift isn't too extreme. Back of the envelope, it'll desynchronise a 25 frame/s 400 line TV picture by one line every 20s. That'd be enough to be annoying, but not enough to stop you watching if you wanted to, and if you are a skilled astronaut you are probably unbothered by it. Joe Public, however, may not be quite so accomodating and NASA may well have chosen to fix the signal before sharing.

That said, I have to say I don't know too much about TV signal formats. It's possible that there are issues receiving a Doppler shifted signal that I don't know about. However, I suspect that NASA engineers are probably capable of creatively breaking a TV so it'll accept a slightly non-standard signal. They can't do that to every household in the world, though.

 

[Dean Talboys]

Whoa! That was fast. The signal received from Apollo was 320 lines @ 10 per second whereas the standard NTSC was 525 lines 60 per second (interlaced so 30 f/s actual) so the effect on the latter is even less than in your example. Thank you!

 

[tech99]

When receiving analogue TV, the time base oscillators are adjustable in frequency over a small range and are held in lock by a phase lock loop. So there is little problem if the received signal is, in effect, referenced to a slightly lower frequency. The timing relations between frame and line remain unaltered. For reception from the space craft, I think the transmissions were made using slow scan, so I imagine the twin VTR set up was to provide standards conversion from slow scan to standard fast scan to allow viewing by public TV receivers. Slow scan was usually displayed on a long persistence tube at that time. Slow scan can vary in its standard but may be in the order of 60 lines per second and 120 lines to a picture.

 

[berkeman]

When receiving analogue TV, the time base oscillators are adjustable in frequency over a small range and are held in lock by a phase lock loop. So there is little problem if the received signal is, in effect, referenced to a slightly lower frequency.

That was my thought as well.

I was trying to remember what caused the vertical phase to vary for some received signals. I seem to remember that making small frequency tuning adjustments allowed the vertical synch to lock up, but I'm not sure that was the adjustment. It probably happened for very noisy signals where the PLL was having trouble locking.

Is anybody else old enough here to remember making that adjustment to vertically lock the picture?

 

[tech99]

Yes I am very old. The vertical oscillator (frame timebase) was normally adjustable, and would lock when close to the required frequency. On early receivers it locked by injecting sync pulses into the actual oscillator, which tripped the flyback just at the right point, and on later receivers by a phase locked loop, sometimes called flywheel sync. The latter was more resistant to disturbance by noise pulses such as those from the ignition system of passing vehicles, or just thermal noise on a weak signal. By the 60s the system worked very well and was not prone to slipping, especially when in the UK where we changed to 625 line UHF monochrome, using positive modulated syncs.

 

[tech99]

Regarding the Doppler Shift of the actual radio signals, I don't think the astonauts were watching terrestrial TV, but pictures on a microwave uplink. Such a system would likely use FM and occupy maybe 20 MHz. Therefore a few kHz shift due to Doppler would be insignificant. The space craft antenna is looking at a noisy Earth, but the transmitting station can use several kilowatts into a high gain antenna. For the down link, the spacecraft is likely restricted in transmitter power, but the Earth Station is now looking into a cold sky and slow scan can be transmitted in audio bandwidth, reducing noise. In this case, however, it would likely be necessary to compensate for Doppler Shift.

 

[jbriggs444]

Is anybody else old enough here to remember making that adjustment to vertically lock the picture?

Yup. "horizontal hold" to stop the picture from tearing loose from the edges in a curve that got worse toward the bottom of the screen. "vertical hold" to stop the picture from rolling.

Then there was the central dot that remained illuminated after the set had been turned off.

 

[berkeman]

Then there was the central dot that remained illuminated after the set had been turned off.

Hah! I seem to remember a circuit called a "beam killer" or similar that was used to speed up the extinguishing of that bright dot. It may have been for B/W CRT computer monitors that I worked on at one point back in the 1980s rather than for commercial TVs, though.

 

[Dean Talboys]

When receiving analogue TV, the time base oscillators are adjustable in frequency over a small range and are held in lock by a phase lock loop. So there is little problem if the received signal is, in effect, referenced to a slightly lower frequency. The timing relations between frame and line remain unaltered. For reception from the space craft, I think the transmissions were made using slow scan, so I imagine the twin VTR set up was to provide standards conversion from slow scan to standard fast scan to allow viewing by public TV receivers. Slow scan was usually displayed on a long persistence tube at that time. Slow scan can vary in its standard but may be in the order of 60 lines per second and 120 lines to a picture.

The scan conversion consisted of three components - twin VTRs sharing a tape to allow for Doppler, a TV set to display the incoming 320 line 10 frame signal from Apollo with (I think) a standard NTSC 525 line TV camera recording that picture along the lines of a kinescope, and a magnetic disc video recorder to hold the frame long enough to rebuild the 60 frame interlaced output required by network TV. I haven't been able to find any graphics on how the tape system worked, only a description that the first VTR (recording) was sync'd with the incoming signal and the second VTR (playback) was sync'd with the NTSC output. This appears to me to be a hardware solution to buffering to allow for the 'too fast' or 'too slow' signal due to the Doppler effect, which implies there is a degree of slack tape between the two VTRs running at different speeds. Does that make sense? If so I guess there would be a time when they have to pause the process to allow one or other VTR to catch up.