Videoquip Research Limited Tec Tips

Reference Technical Information

Propagation Delay Through RG-59 and Belden 8281

Video Level Relation Between IRE Units, Volts and dB

dB = 20 * Log (V₂/V₁) where V1 = 1 Volt, V2 the voltage measured.
IRE Units = 140 * Voltage

IRE Units	Volts	dB

140	1.000	0.000
139	0.993	-0.062
138	0.986	-0.125
137	0.979	-0.188
136	0.971	-0.252
135	0.964	-0.316
134	0.957	-0.380
133	0.950	-0.445
132	0.943	-0.511
131	0.936	-0.577
130	0.929	-0.644
125	0.893	-0.984
120	0.857	-1.339
115	0.821	-1.708
110	0.786	-2.095
105	0.750	-2.499
100	0.714	-2.922
90	0.643	-3.838
80	0.571	-4.861
70	0.500	-6.020
60	0.429	-7.359
50	0.357	-8.943
40	0.286	-10.881
30	0.214	-13.380
20	0.143	-16.902
10	0.071	-22.922
9	0.064	-23.838
8	0.057	-24.861
7	0.050	-26.020
6	0.043	-27.359
5	0.036	-28.943
4	0.029	-30.881
3	0.021	-33.380
2	0.014	-36.902
1	0.007	-42.922

Propagation Delays of RG-59 and Belden 8281 Coax Cables

Speed of Light	299,792,458 meters per second
	0.984 feet per nanosecond
RG-59 Propagation delay	66%
Belden 8281 Propagation Delay	78%
Delay of signal through RG-59	1 nanosecond per 0.65 feet of cable (19.81 cm)
Delay of signal through Belden 8281	1 nanasecond per 0.77 feet of cable (23.4 cm)
1 degree NTSC (3.58 MHz)
1 degree PAL (4.----MHz)

Time nsec	Length of RG-59 in ft	Length of 8281 in ft.

1	0.649	0.768
2	1.299	1.535
3	1.948	2.303
4	2.598	3.070
5	3.247	3.838
6	3.897	4.605
7	4.546	5.373
8	5.196	6.140
9	5.845	6.908
10	6.494	7.675
20	12.989	15.350
30	19.483	23.026
40	25.978	30.701
50	32.472	38.376
60	38.966	46.051
70	45.461	53.726
80	51.955	61.402
90	58.450	69.077
100	64.944	76.752
200	129.888	153.504
300	194.832	230.256
400	259.776	307.008
500	324.720	383.760
1000	649.440	767.520

dBfs, VU, PPM, dBv, dBu, dBm

dBm

The dBm is defined as the power dissapation of 1 mW into a resistive load of 600 Ohms. Given a power source, a source resistance and the load resistance, the relationship is as follows:

dBm = 10 * log (P₂; / P₁) where P₁= 1 mW (0.001 Watts) P₂ the power in question.
So if we measure a voltage across a 600 Ohms Resister as 2 Volts RMS, than we could calculate the dBm equivalent as follows:

P = V*I or P = V ² / R

P = 2 ²/600 = 0.007 Watts

dBm = 10 * log (0.007 / 0.001)

dBm = 8.451 dBm
In the same manner, but working backwards, if we are told that we have 10dBm and wish to measure this with a standard RMS meter we would first have to calculate the RMS value as follows:

10 dBm = 10 log (P₂ / 0.001)

1dBm = log (P₂ / 0.001)

10 = P₂ / 0.001

0.01 = P₂

Therefore the power across the 600 Ohm resistor is 0.01 Watts.

Again P = V ² / R

0.01 = V ² / 600

6.00 = V ²

V = 2.449 Volts RMS

It should be noted that the dBm is always into 600 Ohms and the source resistance use to be 600 also but now it is often low 20 - 60 Ohms
When the source is 600 Ohms the generator has to supply 2x the power as half will be dissappated through the source resistance. With low source resistance less power will be consumed at the source. What is important is that for 10 dBm the voltage across the load is 2.449 Volts RMS regardless of source resistance.

It should also be pointed out that 2.449 Volts into 500 Ohms or any other resistance is NOT 10 dBm.

dBu

dBv

VU and PPM

VU and PPM are grouped together because they essentially do the same thing, They both monitor audio levels. VU (Volume Units) meter displays the average volume level, and the PPM (Peak Program Meter) displays the peak volume level of audio.

When a continuious sine wave is applied , the difference between VU and PPM is about 3dB. However for complex audio signals the difference can be as high as 12 dB. This difference is refer to as the crest factor.

The ballistics of the meter are also different between VU and PPM. When a 1 kHz tone is fed to the VU meter it takes about 0.3 seconds for it to stabalize. The PPM takes about 0.01 seconds.

Since the ballistics are different and the crest factor the two meters will display the audio in a very different way.

The VU was designed to closely reflect the sensing level of the human ear. On the other hand it is a poor indicator of peak signal levels because of its slow response. Normally the VU meter will indicate the peak signal about 8 to 20 dB below its actual value.

It should be kept in mind that 0 VU represents the normal operating level for an individual facillity. This could be -10 dBm, 0 dBm, +4 dBm or +8 dBm. The VU meter can be calibrated so that 0 VU can be any level.

dBfs

dBfs is commonaly used in digital applications and it refers to how far the reading is from full scale (fs). The full scale in this case meaning the maximum level that the ADC can convert.
The normal operating level can be -18 dBfs, which would give you +18 dB of headroom before the ADC goes out of range.
The operating level of -18 dB varies at different facillities. The important thing here is to know what it means.

When using digital meter or analog meters for that matter, keep in mind that these devices are usually calibrated to give you a high resolution near operating level. As the level gets further from the normal, the resolution drops. For instant near operating level one step on the display may represent one dB, however one step in the display at levels near -50 may actually be 10 dB or higher.

Glossary

A-weighting filter: a noise filter with specific characterestics (ANSI S1.4, IEC Recommendation 179) used to produce noise which correlate to the human obervations.