Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!att!pacbell.com!mips!samsung!uunet!iconsys!jensenq From: jensenq@iconsys.icon.com (Quinn Jensen) Newsgroups: comp.dsp Subject: Re: Digital Output and VU meters Message-ID: <1991Mar14.033732.18845@iconsys.icon.com> Date: 14 Mar 91 03:37:32 GMT References: <50146@apple.Apple.COM> Organization: Sanyo/Icon International, Inc., Orem, Utah Lines: 157 From article <50146@apple.Apple.COM>, by malcolm@Apple.COM (Malcolm Slaney): > What does 0VU really mean? I assume it is power related but I'm not sure of Here is a short article a friend of mine wrote about VU and audio calibration in rather lay terms but I think it answers the question. Please direct comments to the author. ------------------------------ cut here ------------------------------- Recording Standards - Background Information & Terminology ---------------------------------------------------------- by Brian Minert (hellgate.utah.edu!uplherc!wicat!meph!minert) Most of our present-day audio standards work was done over 50 years ago, and we owe much of the work to the telephone and movie/recording industries. dB Measurement -------------- For example, while Harvey Fletcher was working for old Bell Telephone, he developed technology for carrying phone conversations to long distances by using repeaters at specific intervals along the way. The length of the interval was determined by the audio level, and when the audio power was no longer high enough, it had dropped "1 Bell" and had to be boosted by a repeater! It was soon found that dividing the unit into 10ths became easier to work with, hence the deciBell (dB) was born. As a unit of measurent, the dB is RELATIVE, NOT ABSOLUTE. For example, a 3 dB increase indicates doubling the power, while a 3 dB decrease represents reducing the power by half. Test Milliwatt -------------- In order to test and calibrate various instruments, a standard known as the "Test Milliwatt" was adopted in 1939. It is essentially a 1000 Hz tone across a 600 Ohm resistor such that 1 milliwatt is dissipated by the resistor (That calculates to a voltage of about .7746 Volts RMS). Again, this was a telephone company standard that grew to wider application. The great value in the Test Milliwatt reference is that it defines an absolute power level. dBm Measurement --------------- Now we have the relative measure of power, the dB, and an absolute power reference level, the Test Milliwatt. Industry chose to measure power levels in dB, using the Test Milliwatt as the reference. Enter dBm! When measuring a Test Milliwatt, the result is 0 dBm, or 0 dB referenced to the Milliwatt (Test Milliwatt). Thus measurements in dBm represent absolute levels, and therefore are of the proper form to be used as calibration and recording standards. Power Level Calibration ----------------------- The history of audio calibration involves the term "100% modulation", or "how much power can I get out of this thing?". High-end power is therefore used, while still avoiding major distortion. Please be aware that a CALIBRATION standard is NOT the same as a RECORDING standard. Calibration is performed on the recording equipment, after which the recording takes place at the level APPROPRIATE TO A SPECIFIC MEDIA/TECHNOLOGY. For example, reel-to-reel mag tapes are often recorded at +4 dBm, while digital CVSD might be recorded at -10 dBm. Recording Standards - Page 2 Distortion ---------- I just used the term Distortion without defining it. Here goes! When the original analog waveform shape is altered from its' true form, it is called distortion. Some examples include: Clipping Distortion - the waveform is amplified so high that the signal peaks attempt to go beyond the amplifier voltage range. The result is a plateu (flat top) effect on a sign wave. Phase Distortion - electronic components such as capacitors and inductors respond differently at different frequencies. When such effects are prominent, the audio signal is slightly delayed in a frequency-dependent manner. Proper design and proper connections between equipment avoid phase distortion. Noise Floor and Distortion Threshold ------------------------------------ How is the appropriate record level decided? Each technology has a level of silence. Examples include a blank tape, uLaw PCM FF pattern, etc. When "silence" is played, there is always "hiss" that can be heard and measured at some low level. That level is known as the Noise Floor. Floor noise is caused by imperfections in the power supply, amplifiers, etc. For good audio equipment, the noise floor will be far below -70 dBm. In reversal, there is some maximum input level that can be tolerated, above which clipping (sine wave peaks are flattened on the top) or other types of distortion become significant. This is called the Distortion Threshold. For example, telephone levels must be limited to +6 dBm, and even that is unpleasant. Signal-to-Noise Ratio --------------------- We are now left with some number of dB between the Noise Floor and the Distortion Threshold. It is between these two levels that usefull recording takes place. This number of dB between the two levels is called Signal-to- Noise Ratio. For most technologies, this full range cannot be used. Dynamic Range ------------- For most technologies, there is a "lowest record level". In PCM, that is heard by alternating between FF/7F (silence) and FE/7E (almost silence). The highest level is heard by using values of 00/80. The difference between these two levels is the dynamic range, meaning the range over which there can be level change. Dynamic range is almost always less than the Signal-to-Noise Ratio, because recording levels need to keep well away from the Noise Floor and Distortion Threshold. Recording Standards - Page 3 dBm Recording Level and Headroom -------------------------------- Recording Level (in dBm) represents the average power level seen during an audio sample (typically of 10 seconds length). A Record level is chosen for a particular media by choosing a level far enough above the lowest record level (which is already well above the noise floor) to record quiet sounds, yet leaving much of the available dynamic range above it to safely record the peaks naturally associated with human speech. The area between the Record Level and Distortion Threshold is called Headroom. The actual value chosen for recording a particular media or technology is then based on experience and best judgement. VU Meter -------- One of the most popular instruments ever used while recording audio is the VU meter. It was in use as early as the 1920's, and the movie industry was using it by the 1930's. It is a mechanical device meant to respond in a manner similer to the human ear, and displays an indication somewhere between peak and average for complex (speech/music) waveforms. VU is dimensionless, where units of VU refer only to the VU itself. It is a relative measure only, and is used in the following manner: In a recording studio, the calibration engineer will adjust calibration (calibration = "hidden and not to be touched") levels FOR THE PARTICULAR MEDIA TO BE RECORDED ON, such that the proper dBm level is being recorded on the particular media when the VU meter is "bumping" on 0. The calibration engineer then locks the cabinet. The studio recording technician now uses the studio for recording audio. He adjusts mixer inputs such that 0 VU is his goal, based on judgement and experience. Again, VU meters themselves display only relative measurements, not absolute levels. When properly calibrated to a particular media, it will indicate the safe recording zone for studio work. As you can see, it is totally impossible to discuss level standards in terms of VU, because any standard can be met at 0 VU by proper studio calibration!