 dBm (800 or 1000 Hz). Another point is that it was decided that all weighted (dB derived) noise units should be positive (i.e., not use a negative sign). First, remember these relationships: 1 pW = 1 x 10-12W=1x 10-9 m = -90 dBm. A weighted channel has less noise power than an unweighted channel if the two chan- nels have identical characteristics. C-message weighting has about 2 dB less noise than a flat channel; a psophometric weighted channel has 2.5 dB less noise than a flat channel. Figure C.2 may help clarify the concept of noise weighting and the noise advantage it can provide. The figure shows the C-message weighting curve. Idealized flat response is the heavy straight line at the arbitrary 0-dB point going right and left from 200 Hz to 3300 Hz. The hatched area between that line and the C-message response curve we may call the noise advantage (our terminology). There is approximately 2-dB advantage for C-message weight over flat response. If it were psophometric weighting, there would be a 2.5-dB advantage. The dBrnC is the weighted noise measurement unit used in North America. The fol- lowing are useful relationships: 0 dBrnC = -92 dBm (with white noise loading of entire voice channel). Flat response (idealized) C-message response Frequency (Hz) Figure C.2 Flat response (idealized) versus C-message weighting. The hatched area shows how we arrive at approximately a 2-dB noise advantage for C-message weighting. We can only take advantage of C-message improvement for speech telephony. For data transmission we must use flat response. C.7 dB APPLIED TO THE VOICE CHANNEL Think about this: 0 dBrnC = -90 dBm (1000 - Hz toned). Figures C.1 and C.2 show the rationale. Value in (-) dBm = 10 log(pW x 10-9), Value in pWp = value in pW x 0.56, -90 dBm = -2 dBrnC and thus -92 dBm = 0 dBrnc(white noise loading), -92.5 dBmp = -90 dBm (flat, white noise), 1 pWp = -90 dBmp, Value in dBm = 10 log(value in pWp X 10-9) + 2.5 dB, dBrnC = 10(log pWp X 10-9) - 0.5 dB + 90 dB, Value in pW X 0.56 = value in pWp, Value in pWp/0.5