Speed of Light
299,792,458 meters per second
0.984 feet per nanosecond
RG-59 Propagation delay
Belden 8281 Propagation Delay
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.|
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:
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.
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.
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.
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.