Doing it with sound AND pictures

AKA: Television!

I am talking about the type of TV you need an antenna for. I have been installing a few of these low power digital TV transmitters at various places and it is good work.

Anywave MPTV 2.2 KW digital TV transmitter, WCRN Boston
Anywave MPTV 2.2 KW digital TV transmitter, WCRN Boston
LPTV antenna side mounted on tower in Quincy, MA  ERP is 15 KW.
LPTV antenna side mounted on tower in Quincy, MA ERP is 15 KW.

Anywave MPTV unpacking, rolling into building, Ellenville, NY

This transmitter’s dry weight is about 500 pounds, which was a little bit too much for our appliance hand truck, so we built a ramp. They have a nice set of wheels on the bottom, so they roll into place.

Carrier power after pre-correction files updated

There are a few differences in the way things are done. First of all, there is a different set of acronyms:

  • ASI- Asynchronous Serial Interface, format for MPEG transport stream, max speed 270 Mbps.
  • SDI – Serial Digital Interface – Similar to ASI but can run much faster, up to 12 Gbps.
  • TS – Transport Stream, Encoded video and audio streams into the exciter. Same idea as composite audio input on an FM exciter.
  • TSID – Transport Stream ID, a unique number assigned to each DTV station and encoded with the transport stream.
  • PSIP – Program System Information Protocol, carries program and system data about the transport stream.
  • ATSC 3.0 – recently updated ATSC standard that allows TV stations to do more with their transport streams than before. Will also change the modulation from 8VSB to COFDM.

Then, some things that look familiar are called by difference names, BNC cable vs ASI cable… I am a neophyte to the TV world, so there still many things to learn.

EAS is still EAS, but now there is locally generated video to go along with the audio.

A while back, some fool wrote their congressman because THE COMMERCIALS ON THE TEE-VEE WERE TOO LOUD, so there is something called CALM Act compliance.

However, at the output connector on the exciter, through the amplifier, filters, directional coupler, etc; it is RF and behaves like RF. Even more interesting; Mr. Doherty’s name is used to describe the RF amp. Doherty amplifier or Doherty modulation was designed by William Doherty for Western Electric in 1936. It was not until Continental Electronics began using it in there AM (317B) and Shortwave (420A) transmitters that it became known broadly. Old things are new again:

Continental Electronics 420A 500 KW Shortwave transmitter, Greenville site B

That picture is from my visit to Greenville, NC in 2017. This is the control console, the transmitter is behind the glass and takes up half the building. It was installed when the site was built in the early 1960’s. The new Doherty UHF amplifiers use LDMOS devices, notably the BLF888E in the Anywave units.

Antenna sweep, channel 30 UHF slot antenna, WCRN Boston
Antenna with channel band pass filter, Channel 30 WCRN Boston

The Channel Band Pass Filter is required by the FCC, basically it ensures that the TV transmitter is staying in it’s allotted 6 MHz channel. They add phase rotation, as noted above. This is why pre-correction is needed to keep the ATSC signal linear across the entire 6 MHz bandwidth. RF is RF and we like RF.

5 thoughts on “Doing it with sound AND pictures”

  1. “A while back, some fool wrote their congressman because THE COMMERCIALS ON THE TEE-VEE WERE TOO LOUD, so there is something called CALM Act compliance.”

    It’s a bit more nuanced than that…

    TV networks and stations had been getting in the habit of doing something to make their paid commercials a lot more attention-getting. And that was, to force the listeners to have to listen to those commercials at a higher volume than the rest of the programming.

    It wasn’t obnoxious at first, but over time, there was increasing pressure by the advertisers to make some commercials stand out even more from the others (read, their competition).

    It got to the point where folks started complaining louder and louder to the lawmakers, and that’s when they passed that law.

    If the advertisers hadn’t pushed the issue by upping the volume to ridiculous levels, they never would have needed to pass that damn law.

    (Note – Some TVs were far worse than others by having a shoddy AGC for their audio, making the ‘blasting’ effect for more noticeable…)

    In my opinion, it was a legitimate gripe by the citizens…

  2. Gregg; Thanks! It is fun to learn new things!
    Geoff; That was intended to be a *slight* bit of sarcasm.

  3. My “day job” is high-band VHF at 4.3kW TPO. Two cabinets of the original 4-cabinet 1996 Harris Platinum solid-state analog TX, converted to DTV. No IOT’s, no crowbar “kabooms”. Keep it cool and clean, and it just works.

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