Radiovibrations.com - Offset DX

The spectrum of an analog TV signal

When you're into Offset DX you need to know some basics about the carriers you can detect. Each analog TV signal consists of multiple signals. The strongest one is the main video carrier (the nominal frequency). But there are other lines in the spectrum which arrear periodically and much weaker. When DXing it's essential to decide whether a weak detected carrier belongs to a completely new station or is just a sub-carrier of a stronger one (often +/- 15625 Hz, with offsets appearing on 12m, 4m, 4p or 12p).

nominal offset

First let's look at the channel structure. Every station has a nominal offset. These have 1/12 steps of the line frequency. The line frequency is the line number * images per second. (see Wikipedia) The default example is the channel R1 (East European / Russian System) on 49.75 MHz with 625 lines and 25 pics persecons which results in a line frequency of 15625 Hz. The offset steps are about 1302.083 Hz. Most common offsets are 0, 8p (8 plus) and 8m (8 minus).
Frequency: MHz lines: pictures per second: line frequency: 15625 Hz offset steps: 1302.0833333333 Hz
offset
name
offset
shift Hz
frequency Hz
8m-1041749739583
7m-911549740885
6m-781349742187
5m-651049743490
4m-520849744792
3m-390649746094
2m-260449747396
1m-130249748698
0049750000
1p130249751302
2p260449752604
3p390649753906
4p520849755208
5p651049756510
6p781349757813
7p911549759115
8p1041749760417
So much about theory. But most transmitters are not as precise as it is suggested here. The coordinated offset can only be seen as a rough indicator. Some transmitters drift and warble. Even the precise ones are often a few Hz off. For DXers this is a revelation: It offers the possibility to distinguish between them.

color and audio

Let's start with the color and audio. Their position depends on the TV system. Here is a rough view of the most important ones (which are still in use).
systemcolor
modulation
color
offset MHz
audio
modulation
audio
offset MHz
level
NTSC / AmericanQAM3.58FM4.5
PAL / West EuropeQAM4.43
(supressed)
FM5.5-12 dB
SECAM / East Europe / ChinaFM4.25 / 4.40625FM6.5
Stereo audio can be modulated on 5.7421875 MHz (FM, -18 dB) or 5.85 MHz(NICAM digital). NTSC uses MTS which is modulated upon the FM audio just like radio-FM but with a different pilot tone and spectrum (pilot at 15.734 KHz instead of 19 KHz).

video sub signals

Now the interesting part. The sub signals of the video (whether the definition sub-carrier is correct or not won't be discussed here).

line frequency

These are signals +/- 15625 Hz away from the main carrier. They are much weaker. The first one can easily be mixed up with other main signals. If you have a strong signal on 8p for example, a sub signals will appear at 4m with -30 dB.
line carrier #frequency Hzstrength
dB relative to main carrier
-549671875-35
-449687500-30
-349703125-35
-249718750-25
-149734375-20
0497500000
149765625-20
249781250-25
349796875-35
449812500-30
549828125-35
649843750-35
749859375-35
849875000-35
949890625-40
1049906250-40
1149921875-40
1249937500-40
13 .. 30.. 3 KHz< -50
> 30< -60

AC frequency

As a matter of fact many stations have a modulated AC noise at harmonics of 50 or 60 Hz, a result of unclean modulation.

Real DX

So now imagine a few signals appearing at once and to sort the spectrum - here during a sporadic E event on channel R1 received in Handewitt, Germany in 2008. This is a quick analysis:
49.739.500 8m Lvov?, strong with patterns
49.740.550 8m unknown
49.740.600 8m unknown
49.744.775 is 49.760.400 - 15.625
49.747.400 2m Moscow, only weak MS scatters
49.749.975 0 Petersburg
49.750.000 0 Minsk
49.750.005 0 Cahul, Moldova?
49.755.125 = 49.739.500 + 15.625 with the patterns
49.757.800 6p various Belarus/Ukraine/Russia possible
49.760.400 8p unknown
Forget about the constant lines. They are locally produced by power supplies or other devices. Another impression of a low level investigation is my analysis of the permanent meteor scatter receptions on channel R1 on an ordinary afternoon.