Greeting earthlings. If you follow the channel, you know that we love all things Apollo,
and that during our last visit to Steve Jurvetson’s amazing space collection,
we were given the opportunity to take two holy boxes of Apollo electronics to our lab.
These are the boxes that brought you voice, data and live TV from the Moon, and should be
early masterpieces of microwave electronics, the blackest of black arts in analog electronics.
Over the previous episodes, we got the microwave S-Band link running between the transponder
and NASA ground test equipment. But the microwave S-band boxes, in yellow here,
are just the tip of the iceberg. There are many more boxes that complete the system
and that we need to bring up. And lucky us, we have them, thanks to Marcel.
In episode 17, we took apart the magnificent Pre-Modulation Processor, or PMP, made by Collins.
This box is the heart of the modulation system.
It is the central point where all the many sources of communications converge.
The PMP performs the complicated subcarrier modulation to allow
all these sources to be transmitted simultaneously on the microwave link.
All of the traffic goes through this little box. Voice up, voice down, command up, telemetry down,
dumps of voice and data from the flight recorder, TV, LM relay and more.
And to make matters worse, there are a whole bunch of redundant emergency modes
that makes things even more complicated.
When you see the flight controllers in the MOCR busy with their many screens and audio
and video from the spaceship, try to think that all of that info went through this little box.
It takes two separate NASA drawings just for the simplified high level block diagram.
Here you see how the PMP sits at the center of all the voice connections,
and here you see how it is also at the center of all the data connections.
Once we took it apart, it took Mike weeks of beeping to verify what the pinout was,
and Ken Shirriff studied what each of the modules do, and wrote an article about it on his blog.
And while Mike and Ken were busy, I had the task of figuring out how to create a test rig
for the first power up. This box has so many connections that I had to bring out my giant
box of colored markers to disentangle the wiring. And I still wish I had more colors.
So let me first explain what it is supposed to do.
We’ll need a giant dollop of elevator music for that one!
Let’s look at the audio drawing first.
The PMP sits in the middle, but there is also
a little switch on the side, we’ll come to that later.
On the Intermediate Frequency side, it connects directly to our S-band transponder, as expected.
On the audio side, it connects to the Audio Center, which is essentially an
audio mixer for the 3 astronauts. We do have the box, and it looks like this.
You can clearly see 3 audio connectors, one for each of the astronauts.
It also connects to the Data Storage Equipment, or DSE. This is the flight audio and data recorder.
It’s a monster of a recorder, with many tracks, speeds and features, and sadly we don’t have it.
But we’ll try to substitute for another data recorder. The recorder records audio and data
when not in contact with Earth, and can replay it at high speed when contact is regained.
Now comes the unexpected part. It is also connected to the VHF radios.
Why is that? The VHF radios sure cannot reach Earth during most of the mission. However, the VHF
radios can reach the LM, or an outside astronaut during an EVA. So they are used as relay between
the local radio transmission and Houston for LM voice and EVA voice. More on that in a second.
We’ll skip over the various inputs and outputs, which we will explore when we
dive into the hook up schematics. However, I want to point out the relay one in orange here.
It connects to our PMP relay on the side here.
It’s a simple trick to steal the microphone of the center console astronaut,
who is not here anymore when the relay mode is used. He is
either in the LM on its way to the Moon, or on an EVA outside the CM.
So they reuse the center couch microphone to send Houston’s voice via VHF to the LM
or the EVA astronaut, completing the up direction of the voice relay circuit.
And for kicks, let’s not forget the swimmers umbilical. That’s of course for after splashdown,
so the swimmers can connect to the audio system and congratulate the astronauts.
That’s it for the audio, let’s look at the data block diagram now.
As before, the PMP is at the center of things.
Same connections to the transponder and the DSE.
New box here though, called the PCM in NASA jargon,
for pulse code modulation. I call it the telemetry box or the data box.
We have it too, and it’s another monster box, actually the heaviest of the lot by far,
and that’s the one that floods the control center with hundreds and hundreds of data feeds.
On the side of it, we have the CTE, or Central Timing Equipment.
I just got it, I bought it at the latest RRauction, and spoiler,
we tried it already and it works! That’s the ship clock, used for time stamping the data,
but most importantly, it generates the clocks that the PMP and other equipment rely on.
The updata link box, we have it too,
it's the box that will further decode the updata commands and remote control the spacecraft.
Honorary mention to the SCE, or signal conditioning equipment. That’s the one that
you need to reset, by flipping it to AUX after you have been zapped by lightning in Apollo 12.
Its purpose is to scale analog signals from various sensors
to a standard 0 to 5V needed by the telemetry box.
The G&N box here, which stands for Guidance and Navigation,
is our trusty Apollo Guidance Computer, which both receives commands from Earth and talks back via
the telemetry box. You could dump the entire memory content of the AGC if you wanted to.
This time, the VHF B radio is used to relay the LM data and record it on the DSE, for later playback.
The 3 SCI input on the left of the PMP refers to 3 analog scientific data signals.
NASA had originally grand plans for 9 SCI signals, but scaled them down for the first 3 missions.
Then, they scaled them back up big time for the later J-missions, giving science
it’s very own telemetry system, which was even better then the original one for the spaceship.
Alright, we are done with the overview. But
we need to look at more detailed schematics so we can hook up our PMP.
Maestro, keep up the good work with the background music and let’s switch to game music for a change.
Alright. If we want to power up the PMP, a block diagram is not going to cut it.
We have to dig one level deeper into the schematics.
For simplicity, we’ll ignore the box to the left,
which is part of the extra scientific package that was only present on the later missions.
So we start with the signal coming from the S-Band receiver, in yellow.
If you remember episode 15, the received spectrum has 30 and 70 kHz subcarriers in it.
Here is the carrier,
here is the voice at 30 kHz,
and here is the updata on 70 kHz.
The PMP needs to extract the two subcarriers and demodulate them.
This is done by the Data and Voice Detector Module, which has two two demodulators,
one for the 30 kHz, for the voice, and one for the 70 kHz, for the data.
But there is an extra path, called the backup voice.
This is a redundant mode, in case the 30 kHz path fails. In that case, the updata signal is
ditched altogether, and the voice is switched over to the 70 kHz subcarrier.
A pair of control lines then activates the backup voice path
to switch the audio back to the correct output.
You can also find the sneaky circuit that steals the center couch microphone when in relay mode,
as we had mentioned before.
This circuit will also need its own control line.
Finally, one more line controls the squelch circuit.
This prevents the sudden loud noise to come through if lock is lost.
But sometimes you want to listen to that noise disappear, to know you have lock,
say if you are just reappearing from behind the moon. So there is a switch to disable squelch.
One more thing to note, there is a mystery 400 Hz key tone. But we’ll save that for later.
We are done with the uplink.
So let’s switch to the PM downlink.
Ranging noise turnaround is done in the transponder. But the PMP is responsible
for creating the voice subcarrier, the data subcarrier, and modulating both of them.
The voice subcarrier is created by this module.
The output has two paths. A direct one when using voice only,
and an indirect one when voice and data are both used at the same time.
Which will of course require a few more control lines.
The voice modulator uses the 512 kHz reference from the CTE to create its 1.25 MHz subcarrier.
Let’s now follow the voice input from the astronauts.
It first goes through a clipper, which in modern lingo
is a voice compressor to limit volume variations and promote intelligibility.
There is also a backup mode for the downvoice,
which dispenses with subcarriers altogether, and goes straight to the transponder baseband.
Here too, you lose the PM data, but you keep voice contact with the astronauts. And as the flight
controllers would discover on Apollo 13, there is even a sneaky way to flip the data over to FM.
And as usual that will cost you a few more control lines.
Next we come to the voice relay module.
This one is for the LM relay, and mixes in the LM voice coming from the VHF radio.
It also takes the intercom from the CM and adds the LM voice to it.
So now the LM and the CSM are on the same intercom as if they were still together.
And you guessed it, that will cost you another bunch of control lines.
OK we are through, we have our voice on our 1.25 MHz subcarrier.
So now, on to the PM data path.
The data subcarrier is created by this next module.
It uses biphase modulation, or BPSK in modern terms, to encode the data on the subcarrier.
It takes the bitstream from the telemetry box as an input, and another 512 kHz reference,
this time from the PCM box.
The data output gets added to our previous voice signals.
And by now you probably expect an emergency or redundant mode.
And you are right! There is an other similar modulator, called the auxiliary modulator.
In case the primary modulator fails, it takes the same inputs
and produces the same outputs. And in a sneaky way, it also leaks it into the FM channel,
which is how the Apollo 13 folks kept data going in the downvoice backup mode.
Which brings us to the power supplies,
of which there are of course two for redundancy, the main and the auxiliary.
To simplify control, switching the power supplies to Aux also
switches the modulator to Aux at the same time.
A few more control lines to adjust modulation depth for the various modes,
and we are done with our real time data.
Let’s move on to the FM downlink. That’s where you’ll find the telemetry playback, from tape.
So, in regular operation, the Aux Modulator is not used for real time data.
It is used instead for CSM data playback from the tape recorder.
And the Aux modulator main output goes out to the FM downlink.
We are making excellent progress, we have now added playback CSM data over here.
Now we just need to add playback for recorded CSM and LM voice as well as playback for LM data.
This is done directly on baseband.
So the signals are just replayed a 32 times the original speed,
and both are summed on the FM output.
OK, done with that, but there is a little blank space in-between the blue and the pink. Couldn’t
we use that for something else? Yes we can! What about putting scientific data in that hole?
And there is a modulator for that of course,
with 3 subcarriers and 3 analog inputs from the scientific instruments.
And of course, more control lines.
But at this point, NASA got overwhelmed, just like
us. Why not go to the Moon first and do science later. So they apparently left these unused.
It is unclear if they used it in any of the early missions,
but we should see the subcarriers in our PMP.
But, let’s not forget, we are not fully satisfied by all this yet, we want live TV from the Moon.
So here is our TV camera.
It shoots across the PMP without much processing at all.
There are two outputs for the TV, one to the transponder, and one to the ground support
umbilical, so they could do TV while they were rehearsing on the pad.
Here is our TV on the FM spectrum. This picture is for the LM, which is a bit complicated,
but the CSM is simpler, the yellow part simply replaces all the other FM stuff when the TV is on.
Finally, there is one more important thing, very much a last ditch option:
the morse key, or “key mode" as they call it, with its associated control line.
It bypasses just about everything,
and turns this incredibly complicated system back into the most primitive radio possible.
Each of the astronauts has their own key, which is the push to talk button on their suit.
And that explains our 400 Hz side tone from the beginning. It’s not transmitted,
but it’s mixed in the astronauts audio to let them hear their morse code.
So, phew! We identified all the signals and all the modes, we are now ready to wire up our PMP.
With its million of inputs and control lines, it’s going to be quite a challenge.
So for the power up, we are going to try a minimum setup,
see if we can wire just enough control lines to get some of the subcarriers going.
So, we sort of ran out of time for this episode because of the long explanation,
but there is really no other way to do the PMP justice - it’s just complicated.
And if you want to go one more level down and understand it at the transistor level,
there is always Ken’s blog, linked in the doodly-doo.
So see you very soon in the next episode for the power up of the PMP.
[Mike] R, S, T, U, V... Alright, here we go!
Wow!
That's a signal!
[laughter]