Saturday, October 31, 2009

Happy Halloween!

Wednesday, October 28, 2009

DVB-S on the spectrum analyzer

Here's what DVB-S looks like on a spectrum analyzer:

This is with the modulator configured for a 4500 kilobaud signal, which results in a 6 MHz wide channel. With a FEC rate of 3/4, that's about 6 MB/sec or so of available MPEG bandwidth.

Compared to 8VSB, the envelope isn't quite so square - it rounds off a bit on either end. And, of course, there is no pilot at the bottom end like with 8VSB.

This is another SA view - this time it's calculating the channel average power and the adjacent channel power. Taking into account the 20 dB attenuator on the input of the SA, it's seeing a channel average power of about 4 watts, and the adjacent channels are 30 dB down from there.

N6QQQ/R bench test results

The receiver and my uplink DVB-S modulator have arrived from Germany. With the DEMI 2330PATV, I'm able to get about 4 watts of DVB-S power output. When I started bench testing the repeater, however, it turned out that even with the amp's bias turned off, the DVB-S exciter alone was enough to be received by the receiver. Not too much of a surprise, given that the exciter and receiver were about 3 feet apart in the garage.

I made a YouTube video showing a complete round-trip - from my analog camera, through the MPEG encoder, modulated with DVB-S, amplified to 4 watts and transmitted with my 23 cm 14 dBd loop Yagi. Then received on the DVB-S receiver, modulated as 8VSB on 33 cm, amplified to about 15 watts, then attenuated by 20 dB and run through a rubber duck antenna. Then received on the loop Yagi on the roof, downconverted from there to TV channel 3, and then received on my portable ATSC TV.


The next step will be to start separating the up/downlink station and the repeater further and further apart. Hopefully, the last test will be with the repeater at its new home on the hill!

The next big demo will be at the K6BEN ATV luncheon on November 23rd. I'll schlep all of the gear for both the repeater and my uplink station over to Harry's Hofbrau and we'll try a full round-trip across the parking lot.

Monday, October 26, 2009

Trivia: four-corner counties

There is only one place in the U.S. where four states meet.

But what about counties?

In California, there are two such spots. The two sets of four counties in question are:

1. San Luis Obispo, Monterey, Kern and Kings


2. Santa Clara, Alameda, San Joaquin and Stanislaus.

There are a couple more that might appear to qualify, but if you look at them closely on a map, they don't quite meet up (merely instead forming two nearby spots where 3 counties meet). For it to truly be a four-corners meeting, two map lines must cross each other cleanly (not necessarily at 90 degree angles, as they do at Arizona, Utah, New Mexico and Colorado). While it is possible to make a map where more than 4 areas come together to meet at a point, there are no "5 (or more) corner" spots in California.

So, there you go.

NFL: feast and famine

So what's happened to football? Week 7 just went by (yes, there's one more game tonight, but I don't think it will have any impact on my point) and there are still 3 undefeated teams! The Broncos, Colts and... The Saints?! And to go along with that, there are still two oh-fer teams: the Rams, and... the Bucaneers?! The Lions managed to win one, so this ends motor city's dream of a repeat.

But when the standings just shy of half way through show such disparity between the top and bottom, something about the competitive balance isn't tuned quite right. This isn't supposed to be Pop-Warner, and people shouldn't seriously ask whether or not a mercy rule is appropriate for the NFL.

Why didn't they schedule a Lions-Raiders game? I'd have loved to be a fly on the wall of the CBS sports scheduling meeting to hear them argue about who would have to go cover that game. Bonus points if it would have been in December and in Oakland (meaningless, cold, outdoors, maybe 10,000 really scary Raiders fans in attendance, unquestioningly blacked-out... What a hoot!)

PLL design

The North Country Radio downconverter kit arrived today. It's a bit daunting, but they did supply some suggestions on how to tap off the RF sample for the PLL and put in the VCO control voltage. That was nice of them.

It's hard to find PLL components anywhere - certainly there aren't any one stop shops anymore. Makes me wonder how companies prototype stuff like this in this day and age.

Jameco still sells the MC145151 in DIP packaging, which is convenient.

The North Country Radio downconverter schematic says that the VCO control voltage is 2 to 8 volts. That exceeds the 5 volts that run the MC145151 PLL, which means that the PLL loop filter will need to be an active one. Given that the reference frequency in this case is 15,625 Hz, the op amp for the active loop filter isn't going to have a lot asked of it - an LM741 would do just fine.

That just leaves a divide by 64 prescaler. That, it turned out, was hard to find, and impossible in DIP packaging. I wound up buying a µPB1507GV from Mouser, but I will need to solder it down to an SSOP-8 to DIP adapter board for prototyping.

The nice thing about working on PLLs is that only on the input side of the prescaler are you dealing with real RF - the entire rest of the circuit is dealing with low enough frequencies that it can all be breadboarded.

Sunday, October 18, 2009

More PLL design

So the design at the moment for the downconverter looks like it'll use the MC145151 PLL and the SA620 LNA/Mixer/VCO. At first, I figured I'd use the MC12080 divide-by-80 prescaler. With a 12.8 MHz crystal and a divide by 1024 reference divisor, that's a channelization of 1 MHz. And that's fine for an ATV downconverter.

But what if we altered the design a bit to take advantage of the fractional capabilities of the a dual modulus PLL chip, like the MC145152?

If we used that same 12.8 MHz crystal and divide by 512, we'd have a 25 kHz reference frequency. With a divide-by-64/65 prescaler (like the MC12054A), we'd be able to achieve the same result, but with a 25 kHz channel step.

A PLL that can drive a dual modulus prescaler has two counters. One of them is the actual output into the phase comparator, the other causes a digital output to change state during each count cycle. That signal changes the prescaler from a divide by P+1 to a divide by P.

Let's say that we set the main counter value to N and the prescaler change counter to A and the lower of the two prescaler values to P. What we'd then wind up with is dividing the VCO output by P*N+A. If the reference frequency is R, then the output frequency will be R*(PN+A). So for 848 MHz, if P is 64, and R is 25 kHz, N would be set to 530 and A to 0. Set A to 1 and the output would instead be 848.025 MHz. For 849 MHz (which puts 909-915 down to TV channel 3), you set N to 530 and A to 40.

In general, for a desired frequency F, you set the N to the integral quotient of (F/R) divided by P. You then set A to (F/R) mod P. Stated another way, you're building a fraction of N + A/P, which winds up being equal to F/(P*R).

Of course, this does mean that the design will need more DIP switches. The MC145152 has 6 bits for the A value and 10 bits for the N value. That's two banks of 8 switches. You could fix the top 4 bits of N to 1000 - limiting N from 512 to 575. The resulting frequency range would be 819.2 MHz to 921.6 MHz - more than enough for our purposes. That's a total of 12 switches - two banks of 6.

So a 12.8 MHz crystal, an MC145152 PLL, and an MC12054A prescaler and an SA620 LNA/VCO/Mixer.

Now the big problem is either finding inventory on these parts somewhere or finding equivalents.

Make your own NFL doubleheader!

I'm watching the Giants and Saints game right now. It's on KCBA. Yay!

KTVU's scheduled to air the Cardinals-Seahawks game at 1. Last I heard, the Raiders hadn't sold out, so that game may not air. That would be our punishment, I guess, for not buying tickets to see JaMarcus throw picks and incompletes. Frank Caliendo said it best today - he called the Raiders "Clippers bad." Word.

Anyway, CBS's game today is Ravens-Vikings. So I am flipping back and forth between two games featuring teams off to a 5-0 start. Can't complain much about that! If KTVU indeed airs the Seahawks game this afternoon, well, I'll have gotten to see 3 games instead of 2. And thus, the antenna gambit pays off!

Saturday, October 17, 2009

K6BEN retransmitted digitally

Today I retransmitted the output of the K6BEN repeater digitally over ATSC.

As soon as N6QQQ/R goes live, I'll start doing this on Wednesday evenings just to sort of bootstrap the use of the new repeater - that way folks will have more to look at on it than just my garage.

It's a miracle!

Look! It's Michael Jackson risen from the dead!

Friday, October 16, 2009

Designing a downconverter

I've done some looking into it, and it appears that the three parts that would be the most useful would be the SA620 LNA, VCO and mixer, an MC145151-2 DIP switch controlled PLL, and an MC12080 UHF prescaler. From what I can tell, those three parts should be able to work together to downconvert from just about any frequency in the 33 cm band down to VHF channel 3 or 4.

Yeah, I know. In the last post, I made an argument for putting it in UHF instead. But the coax losses are much, much lower at VHF, so if it's reasonable or easy to make it happen, well, why not?

The big problem is, so far as I can tell, the SA620 and MC12080 are only available as surface mount devices. It'd be nice if they were available in DIP packaging. But, alas, no. SparkFun electronics, however, sells SOIC-8 and SSOP-20 to DIP adapter boards.

The concept is that the RF comes in and the LNA built-in to the 620 amplifies it a little and feeds it into the mixer. Meanwhile, the VCO generates the IF of about 850 MHz or thereabouts. A little of the VCO's output is fed through the prescaler, and then out to the PLL. The PLL then generates corrections for the VCO's voltage.

Those 3 chips total are less than $15, from what I can tell, from a combination of DigiKey and Jameco.

And that's all there is to it: 3 chips, a 5 volt regulator, 2 sets of DIP switches and some discrete components to tie it all together. For extra credit, it would be fairly simple to add a bias-T to the output to supply power to the thing to mast-mount it. And if it's going to be mast-mounted, then DIP switches are a completely reasonable way to set the LO frequency, since it'll be something you only set once.

In fact, if you're using a computer peripheral tuner, like the HDHomerun, you'll likely just set it for an LO frequency of 848 MHz - that would put 902 MHz at 54 MHz. You'd then just tell your tuner exactly what frequency it should use. The only reason for making the PLL as adjustable as it is is because TV sets aren't agile enough to hunt around other than on normal TV channels. The HDHomeRun, however, can easily be told to look for 8VSB on a channel center of 64 MHz - which is where the 909-915 MHz ATV channel would be found.

Thursday, October 15, 2009

Final repeater pieces ordered

All the pieces are coming together.

The repeater will consist of

From SR Systems:

1 DVB-S 23cm receiver board
1 ATSC 33cm minimod

From Downeast Microwave:

1 3370PAHS amplifier

From Ham Radio Outlet:

1 Diamond X6000A 2M/70cm/23cm vertical antenna
1 Comet KP-20 33cm vertical antenna

I'm going to also set up an uplink station for myself. I want to use it at least at first to retransmit the output from the K6BEN repeater. It will consist of

From SR Systems:

1 MPEG encoder board
1 DVB-S 23cm minimod

From Downeast Microwave:

1 2330PATV amplifier

From PC Electronics:

1 TVC-4S 70cm downconverter
1 RCV3 channel 3 NTSC demodulator

The concept here is that we receive standard 70 cm AM TV on the TVC-4S channel 1 down to baseband video and audio. Send that into the MPEG encoder and transmit it as DVB-S on 23cm up to the repeater. The repeater will then retransmit the transport stream over ATSC on 33cm.

I'll receive that with my receive station:

From PC Electronics:

1 TVC-9S 33cm downconverter
1 HDHomeRun ATSC tuner

I'm not sure I can configure the DVB-S modulator to key on and off based on whether or not there's good video going into the MPEG encoder. If it is, then I could make the whole thing automatic. But if my experience with the ATSC modulator is any guide, it won't do that, so I'll have to turn the thing on and off myself. But Stefan assures me that the repeater (the DVB-S NIM and minimod combination) can key on and off automatically based on the receive signal.

Sunday, October 11, 2009

Michael Moore and Me

From what we have been told, Gizmo's toilet flushing antics make a brief appearance in Michael Moore's latest film, Capitalism: A Love Story.

Gizmo has been on TV a lot. But at least so far as I am aware, he has never appeared on TV without someone from the TV show calling and asking permission. And we've never said "no" to anyone (we did tell the staff of The Tonight Show when they called to ask if he could appear live with a toilet on stage that we didn't think it was likely that he'd make a good appearance, since he wasn't trained to flush the toilet or anything, but we didn't outright say "no" to them). The most we've ever asked was to be told where we can tune in to tape a copy of the clip for our own collection.

So the news that Gizmo makes an appearance in Michael Moore's movie was a complete surprise. They didn't call and ask. We would have said "yes" to Michael had he had the good grace to call and ask for permission to use our video in his film. I don't agree with Michael's politics, but I would have been tickled to see Gizmo on the silver screen.

Given everything I know about Michael Moore, I'm not at all surprised. He wants what he wants, regardless of whatever rules (like Copyright law) he has to trample or people he has to hurt in the process. If you look up "asshole" in the dictionary, there's a picture of Michael Moore.

Mike, you owe me an apology. My only solace is that I know that I'm at the end of a very, very, very long line.

Maybe it's you, Al

Over the course of the last, oh, 30 years of near constant suck, the Raiders have had countless coaches (head and assistant) come and go, and moved twice.

Through all of it, the two constants have been Al Davis, and the aforementioned sucktitiude.

The problem is, you can't fire an owner.

Well, the league some years ago did take extraordinary action against Eddie DeBartolo, Jr., but that was after he was convicted in a corruption scandal. Even then, all the league did was suspend him from an active ownership role for a year.

But the prospect of anyone being able to force Al Davis to hand over the team to someone who can simply do a better job is dim.

But they've tried damn near everything else.

Saturday, October 10, 2009

Downconverters for receiving

The PC Electronics TVC-9S is a very nice downconverter, and I'm glad I bought one, but it is a bit expensive. To really make it easy to get started, I think it would be great if something simpler were available.

North Country Radio make a VCO controlled 33 cm downconverter. I haven't tried it, though, and my one worry is that without a frequency counter, it might be tough to tune. It depends on how much tuning slop your ATSC tuner will allow. But on the other hand, their downconverter is a third the price of the PC Electronics one.

One thing I think that could be changed is that these downconverters are designed to shift 33 cm down to VHF low. That means their mix frequency must be very high - in the 800 MHz or so range.

For DTV, I don't think it's necessary to chose such a low IF. If you picked a mix frequency of, say, 421 MHz, you'd wind up with 909 MHz being output on TV channel 17. That would be ideal for around here, because channels 15 through 18 are unused (since 16 and 17 are used for land-mobile in San Francisco).

So, dear readers, if you can design a cheap crystalized or PLL controlled downconverter to go from the middle of 33 cm to the lower end of the UHF TV band, please let me know - I think you could make a lot of hams very happy.

We have a site!

I've negotiated a lease with a land owner in the East San Jose foothills, so as soon as I can get the last of the equipment, North America's first 100% digital ATV repeater will be on the air!

I've set up a a web site for it, with all of the information on what it will take to transmit in and receive from it.

Tuesday, October 6, 2009

JaMarcus Russell

That deaf, dumb and blind kid sure throws a mean football.

Seriously, I'm a Chargers fan and Russell embarrasses me.

Monday, October 5, 2009

Really, Google?

I got to the CalTrain station this morning in time to catch the 7:25 train so that I could get into work a little early. At 7:27, the signs suddenly said that the train was delayed 12 minutes. I thought the prospects for making up that time in route and still catching the 8 AM shuttle bus were dim, so I decided to check Google Maps on the phone to see what traffic conditions were like.

So Google pissed on my back and told me it was raining.

Here's a great example of what I'm talking about. Here's what Google said at one particular point in the trip:

And here's actually what was going on at the time:

Maybe it's just me, but I wouldn't equate a "green" traffic status with 10 mi/hr.

Thanks for nothing, big G.

Saturday, October 3, 2009

Low bitrate MPEG2

I've been using 5 MB/sec for my experimental transmissions, but I have been hearing from Stefan that European hams have had success with 2 MHz wide 2 MB/sec DVB-S. So on a whim, I set my ATSC transmitter to transmit 2 MB/sec MPEG2 video. And the resulting video looked just fine to my eyes. It got a little blocky when I panned the camera around, so with high motion subject matter, it's not enough. But it's clearly enough for the average QSO.

I still think the receivers should be configured for as much bandwidth as the bandplans will allow (6 MHz on 23cm and 4 MHz on 70cm). If for no other reason than it's conceivable that HD pictures could be sent with H.264 inside 6 MB/sec, and someone might want to try that (despite the fact that it won't be compatible with ordinary TVs).

Friday, October 2, 2009

Diamond X6000A

Wow, the X6000A is big! Yeah, the specs say it's 10 feet long, but it just doesn't really come home until you put it together, I guess. It's a couple feet longer than the Comet KP-20, even. And it has much, much longer radials (likely because they have to work on 2 meters).

One of the issues will wind up being how to mount the two antennas. Of course, most of that depends on the site. But despite the cross-band nature of the repeater, it's still going to be desirable to keep the transmit and receive antennas away from each other - either at different elevations on a tower, or 10-20 feet away from each other horizontally. That will obviate the need to set up a 33 cm band-reject filter to protect the receivers.

Operational differences

Users of K6BEN/R will have gotten used to how that repeater works - in particular, the use of 2 meter audio for coordination. I used to think it odd that the audio wasn't carried into the repeater as part of the uplink, but having used the repeater, it's clear that it's a fairly useful way to set things up. Someone other than the station sending video can be the source for te audio.

The first iteration of the D-ATV repeater won't have this ability: it will simply retransmit the MPEG TS uplinked by the user station. This means that coordination will have to take place via a traditional 2m or 70cm repeater. One thing that is planned for the future, though, is a shack camera going into an MPEG encoder. But that still would mean it would be a different stream than the one the user is transmitting. So you could watch yourself on TV, or listen to the 2m audio and watch the shack cam, but not both (unless you either decode both at the same time with a computer or multiple TVs).

On the plus side, however, the audio that's part of the MPEG encoding will be vastly higher quality than the narrowband FM we're used to. That may be useful in the context of events. Certainly, the ability to multiplex multiple programs and (when multiple receivers come), uplink multiple simultaneous streams will be very useful for events.