A Practical Guide To RF In Broadcast: RF System Monitoring

Studio Transmitter Links, remote monitoring, QoS probes, and other OTA signal monitoring solutions… and verifying signal coverage stability.

In addition to their broadcast channel, TV studios and stations have become a jungle of RF signals. The best way to identify, sort and manage all the active RF sources in a facility and find new signals is with a spectrum analyzer.

Monitoring RF systems can be divided into five specific categories;

  • RF compliance with government regulations, such as transmitter power limits and potential interference, including ATSC and ATSC 3.0 standards and signaling compliance.
  • Audio compliance, loud commercials
  • Baseband video, typically measured and monitored by a waveform monitor and vectorscope and adjusted by gain, pedestal, chroma and phase controls and sometimes the camera iris on the source device.
  • IP and network monitoring, including streaming video on the internet.
  • Monitoring and being aware of what kind of RF signals are active in or around a facility or remote site that might cause interference. Monitoring the whole spectrum calls for a spectrum analyzer.

Over the TV years from analog to today, Test & Measurement gear for operators in many TV stations mainly consisted of baseband analog waveform monitors and vectorscopes, VU meters, and a couple of general-purpose oscilloscopes. Waveform monitors and vectorscopes are important tools for content creation, post-production and ingest preparation.

Live production and limited table space call for floor-mounted, knee top-level monitoring of video levels using a familiar WFM fed by a HD HDMI-to-SD analog converter.

Live production and limited table space call for floor-mounted, knee top-level monitoring of video levels using a familiar WFM fed by a HD HDMI-to-SD analog converter.

These ‘dashboard’ tools help the operator properly set levels and colors in scenes within industry limits on a calibrated display and they are particularly useful with color bars for reference. They are like a speedometer and tachometer on a auto dashboard, displaying to the driver objective safe operating limits for the vehicle and engine. The best time to set video and audio levels is as the original content is captured. Once content has been captured and digitized to calibrated industry standards, new issues can be attributed to IP, which requires different types of monitoring gear more common in a maintenance shop or bench.

RF & Visual Compliance

RF output compliance is usually measured with a pair of wattmeters, one measuring the input power to the mask filter, the other measuring the mask filter output power to the antenna transmission line. Visual transmission compliance ensures the integrity and standards conformity of the video and data signals, including ATSC 1.0 diginet channels and all things in ATSC 3. T&M gear specific to examining streams and pipes is important to verify full signal compliance with ATSC A/52, ATSC A/53, and ATSC A/54A, and FCC rules 47 CFR § 73.682 - TV transmission standards. Accurately measuring ATSC 3.0 signals requires a good RF signal. The FCC defines the edge of a UHF DTV signal contour at 41 dBμ. A 40 dBμ (0.1 mV/m) signal is generally considered the minimum strength a station can be received with acceptable quality on most receivers.

ATSC 3.0 is a highly sophisticated scheme that compresses more data bandwidth onto an RF signal than the bandwidth of the actual RF signal. This is accomplished digitally using Physical Layer Pipes (PLPs). A PLP is a logical channel carrying one or more services, with modulation and robustness unique to that individual pipe. There are books covering the myriad details of PLPs but suffice it to say that every PLP needs a Bit Error Ratio (BER) and a Modulation Error Ratio (MER) sufficient for required modulation and reception quality.

Commissioning a new Channel 49 transmitter that couldn’t wait until repack with a TV analyzer revealed key installation and antenna performance details that all became a learning experience after repacking to Channel 22.

Commissioning a new Channel 49 transmitter that couldn’t wait until repack with a TV analyzer revealed key installation and antenna performance details that all became a learning experience after repacking to Channel 22.

The ATSC recently published “ATSC Recommended Practice: ATSC 3.0 Field Test Plan,” available at A/326, “Field Test Plan” (atsc.org), which precisely explains how to test and measure an OTA ATSC signal.

Audio Compliance

Loud ads annoy TV viewers, and the FCC responds to viewer complaints. The Commercial Advertisement Loudness Mitigation Act (CALM Act), meant to eliminate loud TV ads, began enforcement in 2012. The CALM Act calls for maintaining a target level of -24 LKFS. The acronym LKFS stands for “Loudness, K-weighted, relative to nominal Full-Scale.” LKFS represents the amplitude level that is communicated to viewing audience in decibels (dB), but it focuses on the audio elements that viewers recognize the most.

LKFS is significant because it provides an integrated measurement over time. Achieving the -24 LKFS target level often requires increasing high frequencies and dropping lower frequencies across all 5.1 channels, with additional calculations around power averages for each audio file. If all content sources universally used the ‘dialnorm’ standard, dialog levels would be consistent and uniform from channel to channel. The -24 LKFS level is considered ‘dialnorm,’ meaning the dialog level is normalized.

Many products will log, measure, and display LKFS values. Some CALM Act monitoring gear includes aircheck auto-logging that reproduces technically accurate audio levels and can be used to prove LKFS values over time. Accurate CALM Act logging and archiving is the best possible protection from ‘loud commerical’ complaints and FCC loudness investigations.

Baseband Video

TV monitor displays are subjective. Waveform monitors and vectorscopes are scientifically objective measurement instruments and therefore valuable production tools. At one time, a primary use for a waveform monitor was to measure the timing of sync pulses and the width of the burst signal, because in the vacuum tube days, these values could drift and make it impossible to switch inputs without a glitch. Digital TV automatically takes care of the sync pulse timing, but it takes a human to make artistic decisions such as how to use the full dynamic range of a video image for maximum effect.

Waveform monitors graphically display a 1 volt p-p video signal waveform. Top to bottom range is divided into 140 IRE. The bottom 40 IRE is vertical sync, the upper 100 above the vertical sync measured in 0-100 IRE. Zero IRE represents the blackest legal black, 100 IRE represents the brightest legal white. In the analog days, legal black was 7.5 IRE, and the dynamic range was only 92.5% of todays 0-100 IRE dynamic range.

A 1080i HDMI color bar signal converted to SD analog shows zero pedestal, as HDTV should.

A 1080i HDMI color bar signal converted to SD analog shows zero pedestal, as HDTV should.

Not every camera shot needs to include fully black or the brightest white images. It needs to look natural, but content creation and ‘the look’ is the director’s decision. From an engineering perspective, objects in a scene that are compressed into the whiteness or darkness when captured usually can’t be uncompressed from a captured file. It can be compensated for in post, but it’s better to capture everything live and realistically using all the dynamic range available. Encourage the director to use their creative visual magic in post and set the camera iris and pedestal levels for the best possible image for capture.

Vectorscopes display calibrated color markers on a simple scale for hue and color control adjustments. They work best to calibrate with color bars at the beginning and to monitor for illegal colors in content. Typically, in the analog TV days violations of any of these defined level limits would cause a home TV receiver sound to ‘buzz’ and overload transmitters.

IP & Network Monitoring

RF monitoring in IP and networks includes lots of Wi-Fi and perhaps BAS. Both are most easily discovered and identified with a spectrum analyzer.  RF IP and network monitoring include dedicated solutions such as remote QoS probes for remote IP broadcasting. Some remote TV receiver probes are available or adaptable to a PC or blade, but most TV signal verification is done as prescribed in official FCC signal strength measurement techniques, such as height above terrain (20’ above ground level for single story structures). Cluster measurements “should be 9.84 feet apart, if possible.” Take the first measurement with the antenna on a 20’ mast above ground level at the center of the square. Take four more measurements with the same antenna on the same mast facing the strongest signal from the transmitter in each corner of the 9.84 x 9.84-foot square. The FCC allows a use of a horizontally polarized half-wave dipole or multi-element antenna with gain.

Tracking RF Sources

In 2023, nearly every remote device is connected by RF, from garage door openers, auto smart keys and smartphones to Bluetooth, Wi-Fi, and 2 GHz Broadcast Auxiliary Service (BAS). Additionally, some people who may have used Wi-Fi on their phones while at stations and remote sites can cause IP havoc if their cellphone trying to log on to the production system Wi-Fi is set to dynamic IP.

Sweeping the RF spectrum with a spectrum analyzer is the best way to learn what devices are transmitting on airwaves near you and monitor the active spectrum for new transmitters in mere seconds. Spectrum analyzers are available as software with a USB probe for an antenna connection, some offer optional software with templates for FCC measurements. Others are standalone TV analyzers, specifically designed for broadcast engineering RF system commissioning, monitoring and maintenance. A modern TV analyzer combines a TV signal analyzer, a video and MPEG TS analyzer and a spectrum analyzer in one unit and typically cover from 500 kHz to 3 GHz. Some may require an optional digital demodulator for a single DTV carrier.

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