jchristy@hpspcoi.HP.COM (Jim Christy) (08/24/90)
Does 0 dB mean anything across audio compnent families as far as the signal voltage is concerned? To try and answer this I did the following: 1. Played the CBS Test Disc (reference CD) on track 1 which is marked 1KHz 0 dB reference signal. THis was played on my Sony CDP 520ES CD player. I measured the signal output with a scope at the back of the player and found it to be 2.8 volts peak to peak. 2. Recorded the same signal with my Nakamichi 480 tape deck, after adjusting VU meters to read 0 dB. When played back, the signal voltage at the back of the deck was 1.6 volts pp. 3. Recoreded the same signal with my Mitsubishi VCR on the audio tracks, after adjusting the VU meters to 0 dB. When played back the signal read 0.8 volts pp. Summarizing: 0 dB from CD: 2.8v pp. 0 dB from tape deck: 1.6v pp. 0 dB from VCR: 0.8v pp. Is this really what one should expect? It seems the CD player output is too high (and it has no output level control). Both the VCR and tape deck VU controls had to be set very low to prevent pegging the meters. On the other hand, output levels from the tape media do not agree either! When designing a circuit to feed a stereo receiver or amp via the TAPE IN jacks, what is a good 0 dB reference point? Any help or ideas would be appreciated. Jim H. Christy Hewlett-Packard, PCG 974 E. Arques Ave. MS 72UT Sunnyvale, CA 94086 jchristy@hpspcoi.hp.com
vermilye@Oswego.EDU (Jon R. Vermilye) (08/25/90)
In article <490004@hpspcoi.HP.COM> jchristy@hpspcoi.HP.COM (Jim Christy) writes: > >Does 0 dB mean anything across audio compnent families as far as the >signal voltage is concerned? To try and answer this I did the following: > >1. Played the CBS Test Disc (reference CD) on track 1 which is marked >1KHz 0 dB reference signal. THis was played on my Sony CDP 520ES CD player. >I measured the signal output with a scope at the back of the player and >found it to be 2.8 volts peak to peak. > >2. Recorded the same signal with my Nakamichi 480 tape deck, after >adjusting VU meters to read 0 dB. When played back, the signal voltage >at the back of the deck was 1.6 volts pp. > >3. Recoreded the same signal with my Mitsubishi VCR on the audio tracks, >after adjusting the VU meters to 0 dB. When played back the signal read >0.8 volts pp. > >Is this really what one should expect? It seems the CD player output is >too high (and it has no output level control). Both the VCR and tape deck >VU controls had to be set very low to prevent pegging the meters. On the >other hand, output levels from the tape media do not agree either! When >designing a circuit to feed a stereo receiver or amp via the TAPE IN jacks, >what is a good 0 dB reference point? > >Any help or ideas would be appreciated. I don't know how much help it will be, but... 0db can have many different meanings. Professional audio generally uses the dbm. The m stands for a specific load (600 ohms) and a specific wattage (1mw). This produces a reference voltage of 0.775 across the load. To further complicate matters, the standard VU meter built into audio equipment was designed prior to the wide use of amplified meters, so to prevent loading of the audio signal, a series resistor of about 3.6K is added to the meter circuit. This causes the device's meter to read about 4 dbm too low, so that in most studios, 0db on the meter is actually +4dbm. Many organizations use different references for 0db, so what is true for the audio industry is often not so for the phone system, etc. With your specific problem, most consumer equipment does not conform to accurate output levels. I'm sure the CD is recorded at the 0db level, however the output of the player could be almost anything. Most of the time consumer equipment gives about 1v rms output across a high impedance load (>10,000 ohms). You might find a reference in the manual. To check the output of a tape recorder (audio or video) you must use a pre-recorded tape that is carefully recorded at specific levels. This allows the output of the tape to be set. This is then used as the reference for setting the record level. Once the recorder is correctly set up, the levels should be consistant across the proper load. Setting up levels is critical with professional equipment since you are trying to get as close to saturation of the tape (without going over) as possible. Most consumer equipment operates well below saturation, so exact callibration is not as important. (by the way, setting your bias level has an effect on all this, and is a whole nuther story!!) >Jim H. Christy >Hewlett-Packard, PCG >974 E. Arques Ave. MS 72UT >Sunnyvale, CA 94086 > >jchristy@hpspcoi.hp.com Jon R. Vermilye 315 341 2138 Department of Theatre, Tyler Hall vermilye@oswego.oswego.edu SUNY Oswego ..rutgers!sunybcs!oswego!vermilye Oswego, NY 13126 vermilye@snyoswva.bitnet