Signal Guide

Service Description Selection AM-TV

Here you will find information about the selected service, along with measurement examples.

AM-TV Theory & Details
Broadcast
Typical Frequency Range:

Ranges 1, 3, 4/5: 47 – 68 MHz, 174 – 230 MHz, 470 – 790 MHz. Corresponds to channels 2 – 4, 5 – 12, 21 – 60.

General:

AM TV using the “Gerber standard” is being phased out but is still used for terrestrial and cable TV and by some small TV modulators. Being replaced by FM TV, DVB C/T/S. The synch pulses repeated at intervals of 20 ms (because of the existing TV receivers) are typical.

Modulation: AM

Channel spacing 7 MHz in ranges 1 and 3, 8 MHz in range 4/5. AM TV uses residual sideband modulation. The vision carrier is 1.25 MHz above the channel limit, the color subcarrier is 4.43 MHz above the vision carrier. Two FM audio carriers, each with 50 kHz span are located 5.5 MHz and 5.742 MHz above the vision carrier; their levels are 13 dB and 20 dB below the vision carrier, respectively.

AM-TV Measurement Examples
Spectrum
AM TV channel with 7 MHz bandwidth; the lower band limit is set to 174 MHz. This corresponds exactly to channel 5. The vision carrier
(a) at 175.25 MHz, the color subcarrier
(b) 4.43 MHz above (a), the 1st audio carrier
(c) 5.5 MHz above (a) and the 2nd audio carrier
(d) 5.742 MHz above (a) can be seen.
Spectrogram
Shows the same carrier distribution as Spectrum. The device delivers a “snapshot” every 141 ms.
HiRes Spectrogram Full
Shows the same carrier distribution as Spectrum. However, the recording is without time gaps within the 25 ms. The image changes every 20 ms. The synch pulses are clearly visible. The frequency range has been shifted slightly relative to the Spectrogram to make the details clearer to see.
HiRes Spectrogram Zoom
Section of the HiRes Spectrogram Full. The enlarged time scale covering 3.5 ms shows a change of image with clearly visible vision synch pulses. The display resolves individual lines spaced at 64 µs.
Persistence Spectrum
Calculated from the same data as the HiRes Spectrogram Full. Shows the frequency of occurrence over 25 ms which is the same carrier image as Spectrum.
Magnitude
Time domain display using a different setting to HiRes Spectrogram Full: A narrow channel bandwidth (CBW) was selected to suppress neighboring signals. The vision synch pulses at -1.0 to -0.8 ms, the test lines and videotext at -0.8 to -0.2 ms and the first image lines from -0.1 ms can be seen. Longer recording times of up to 24 h are possible in Long-Time Scope mode.
Scope