TV – Page 2 – Engineering Radio

There is a place on Earth called Meddybemps

I might not know that if I hadn’t been there installing a TV transmitter. We installed this GatesAir VAXTE-2 for Maine Public Broadcasting’s WMED-DT.

GatesAir VAXTE-2, WMED-DT Meddybemps, Maine

After the old Harris Platnum transmitter was turned off, the client got a call from the cable company across the border in New Brunswick. Apparently, they take the off-air signal for their cable feed of PBS in New Brunswick.

We also installed a VAXTE-6 at Mars Hill for WMEM-DT.

GatesAir VAXTE-6, WMEM-DT, Mars Hill, Maine

I was reading through the SBE 2023 salary survey and noticed that those engineers who work in Radio and TV make more money than those who do just radio. My experience is that TV is more technically challenging because there are many more building blocks that go into the end product. ATSC has several layers of complexity starting with video and audio codecs. Then there are various transport methods, PSIP (Program information) tables, aspect ratios, degrees of definition, video and audio bit rate considerations, and muxing, which occur before the Transport Stream gets to the exciter.

DTV ATSC 1 modulation analysis; 8VSB eye pattern

One thing I will note, TV is acronym-heavy. There are many combinations of letters and abbreviations. I can work on a list of things that I have learned, but one of the most important measurements for the quality of the over-the-air signal is MER, which stands for Modulation Error Ratio. MER is measured in decibels and low MER usually indicates some distortion problem.

Once the program material hits the exciter, the process is similar but there are a few noted differences. TV transmissions are 6 MHz wide vs. 200 KHz for standard FM. In order to minimize distortion, the signal needs to be precorrected by the exciter for linearity. HD Radio does the same thing to a degree. High-band VHF and UHF stations tend to use slot antennas. These are high-gain broad-banded systems that are generally very simple. The FCC stipulates that spectrum mask filters be used to limit out-of-channel emissions. During the installation process, the filters must be measured and proofed to comply. In addition, the harmonics need to be measured down to -120 dBm because most of them fall in the wireless data and mobile phone spectrum and we know how those folks can be.

Like other segments of the broadcast engineering profession; TV is struggling to find competent technical staff, so if you are willing to learn new things, consider doing some work in television.

Mars Hill also has many of these giant things:

I’ve never seen one up close, and I will say they do make a fair bit of noise when it is windy. I also noticed that air density makes a difference in the noise levels. When it is cooler or more humid, the noise level goes up. There are twenty-eight 1.5 MW GE wind turbines that generate enough electricity to power 18,000 average homes annually. Maine has several wind turbine farms in various parts of the state. I believe Mars Hill was the first, completed in 2006.

Filters for Over The Air Television

Many people are surprised that OTA TV (Over The Air Television) is still a thing. I am here to say that there are lots of TV stations still broadcasting. OTA is alive and well, especially around big cities. To wit; I noticed this older TV antenna on the roof of a transmitter building in Lodi, NJ. Being curious, I connected an ATSC 1.0 TV to the antenna lead in the kitchen. One scan captured 62 TV channels and sub-channels OTA in the NYC market.

Somewhat aged TV/FM antenna pointed at Manhattan

That site is 10 miles northwest of the Empire State Building.

I also noted that the satellite dishes on site have had Terrestrial Interference (TI) filters on the LNBs for many years. Recently, 5G filters were installed as well. Thus, I added a 5G/LTE filter made by Channel Master (part number CM-3201) to the TV antenna splitter. A rescan captured 79 channels. Interesting.

I began ordering TV receiver filters and testing them with my network analyzer. There are many different units made by different manufacturers. The smaller, cheaper units do not have as good performance as the larger, more expensive ones. Go figure.

Here are a few sweeps of various filters:

Channel Master CM-3201 5G/LTE filter. Cut off 608 MHz

Silicon Dust USA LTE LPF-608M. Cut off 608 MHz

There is also an FM band-stop (Channel Master CM-3202), which is effective for blocking out 87 to 113 MHz.

Sometimes I get questions from non-technical readers, thus for the uninitiated; these sweeps are return loss. The higher the line on the right-hand graph, the less signal will get through the filter. A flat line at 0dB means that little or no signal is getting through on those frequencies.

These filters are helpful, especially with inexpensive consumer-grade TV receivers. If you live near an FM transmitter site, then an FM band-stop filter may help, especially with low and high-band VHF stations. If you live anywhere near a cell site (and most of us do) then a 5G/LTE filter will likely help.

Happy cord-cutting!

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

LPTV antenna side mounted on tower in Quincy, MA ERP is 15 KW.

Anywave MPTV unpacking, rolling into the 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 different names, BNC cable vs ASI cable… I am a neophyte in the TV world, so there are still many things to learn.

EAS is still EAS, but now there is a 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 their 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 1960s. 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 its 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.