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).

offset name	offset shift Hz	frequency Hz
8m	-10417	49739583
7m	-9115	49740885
6m	-7813	49742187
5m	-6510	49743490
4m	-5208	49744792
3m	-3906	49746094
2m	-2604	49747396
1m	-1302	49748698
0	0	49750000
1p	1302	49751302
2p	2604	49752604
3p	3906	49753906
4p	5208	49755208
5p	6510	49756510
6p	7813	49757813
7p	9115	49759115
8p	10417	49760417

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).

system	color modulation	color offset MHz	audio modulation	audio offset MHz	level
NTSC / American	QAM	3.58	FM	4.5
PAL / West Europe	QAM	4.43 (supressed)	FM	5.5	-12 dB
SECAM / East Europe / China	FM	4.25 / 4.40625	FM	6.5

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 Hz	strength dB relative to main carrier
-5	49671875	-35
-4	49687500	-30
-3	49703125	-35
-2	49718750	-25
-1	49734375	-20
0	49750000	0
1	49765625	-20
2	49781250	-25
3	49796875	-35
4	49812500	-30
5	49828125	-35
6	49843750	-35
7	49859375	-35
8	49875000	-35
9	49890625	-40
10	49906250	-40
11	49921875	-40
12	49937500	-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.