Peak Programme Meters
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Peak Programme Meters Audio signals in nature can cover an extremely wide dynamic range extending from, at the quietest level, below the threshold of hearing to, at the loudest level, greatly in excess of the threshold of pain! Broadcasters of course do not attempt to emulate this dynamic range, and for entirely practical reasons a very much reduced dynamic level variation must be transmitted. The Listener, or Viewer, will adjust the volume setting in their own home environment so that the Programme is clearly audible above the domestic background noise which will vary from location to location. Invariably Broadcasters restrict the dynamic level range of the transmitted Programme in order to increase the "impact" of the programme audio over this domestic ambience. The dynamic range is restricted by a combination of close- microphone and electronic compression using "dynamic limiters", and in essence, these techniques make the programme's audio sound louder than it would otherwise be. This compression is usually gross on Pop Music channels where the dynamic range is severely limited and often pretty obvious on Commercials too. Measuring audio levels, in a programme production environment, is more complicated than appears at first glance. Programme Audio is subject to the following:- 1) Wide, though restricted, dynamic range. 2) Wide, though restricted, frequency response. 3) Short duration transients 4) Positive and negative signal excursions are often different by many dB. 5) The degree of audio compression will affect measurements. 6) Stereo Phase considerations. 7) The metering must be clear and unambigous. 8).....and other more subtle effects. The characteristics of the measuring device itself totally weight the achieved measurement. It is no use, for example, expecting to measure fast audio transient amplitude accurately if one simply uses a standard Workshop "ac" voltmeter! VU Meters VU, or "Volume Unit", meters are in essence simple ac voltmeters that are connected across the programme audio line to achieve a measurement. The Specification of a VU meter is very precise, with respect to both the mechanical and electrical characteristics required; and the cheap and nasty meters seen on domestic (and some Broadcast equipment!) bear no relationship to the required performance. These are simply dreadful, and best totally ignored. VU meters measure, via a simple half wave rectifier, which is driven from a defined impedance, "loudness". Transient performance is defined, repeatable but dubious. Amazingly, with some current limited output stages, the asymmetric load presented by the VU meter's rectifier has been known to contribute to overall distortion! British broadcasters seldom use VU metering. PPM In simple terms, systems overload. This overload condition is highly predictable, and is invariably accompanied by a dramatic increase in distortion. Many decades in the past, research was undertaken by the BBC that determined that short transient overload produced distortion that was effectively masked by the audio signal itself, consequently became inaudible, and could be ignored. The modulators of the ancient AM radio system have very clearly defined overload thresholds for both the positive and negative signal excursions; and the BBC developed a sophisticated metering system that took account of these monitoring requirements that became known as the Peak Programme Meter (with the British spelling). It was found that a simple black face with white pointer and scale markings (locally illuminated as in the photo above) increased contrast, whilst the fast attack time together with the slow decay time produced by the special PPM drive amplifier, greatly enhanced the ability to measure peak levels accurately over the duration of a programme without fatique or eyestrain. Because audio levels were compressed either manually or electrically, the measurement range of the PPM was fashioned to match the amplitude range of interest. A semi-logarithmic scale was developed for this purpose, and originally valves such as the octal based SP41 and B7G based EF92 (which was much smaller) were selected because their non-linear transfer characteristics matched the required law. An interesting feature of such valve drive amplifiers was that the meter worked "backwards", with the zero deflection being at maximum, on the right hand side of the meter. The more accurate semiconductor "diode & resistor shaping" networks introduced in the 1960s, required the conventional left hand zero meters which are still in use today, where the scale divisions now each represent 4dB. Interestingly, the attack time of even a mechanical PPM could be made significantly faster than it actually is. The Standard adopted specifically permits the fast transients refered to earlier to be somewhat integrated, permitting a slightly higher amplitude of programme to be originated than would be the case if the peak value transient were to be accurately measured, this difference can easily amount to a couple of dB. The advent of magnetic (tape) recording also required the accurate monitoring of peak signal levels, or excessive record flux densities would result in a significant increase in distortion. On quadruplex videotapes, an increase in level of only 4dB would double peak distortion from 2% to 4%! Dynamic range compression affects VU metering and PPM metering quite differently. VUs which indicate "loudness", will increase the average level indicated (even though the peak level remains unaltered); PPMs on the other hand are not affected. The PPM's peak reading is NOT affected in any way , but the instrument will clearly show this reduced dynamic range of programme audio by increasing the average deflection, and not falling back quite so far on the quieter passages. This is a major difference. In Broadcasting the conventional audio Reference Level is derived from the early days of Telegraphy and Telephony where the Reference was 1mW intointo a load impedance of 600 ohmsohms; this was refered to as "ZeroZero levellevel" (= 0dBm ) which most certainly does NOT mean "no signal" , which is quite quite different. This "Zero level" equated to an absolute voltage of 0.7749V RMS, and today 600 ohm source and load impedances are no longer used so this VOLTAGE level is correctly rreferedefered to as "0dBU" or "0dBV.7" . Decibels are used as it enables Engineers to very quickly calculate "gain through a System", by the simple addition of positive and negative dB gain values, which are simple IN/OUT voltage gain ratios and are not calculated with respect to any particular level. On a British PPM, "Zero level" is adjusted to read "4" on the meter scale, which represents 51% deflection. Peak level is 8dB higher than this (=2 scale increments of 4 dB), and so reads "6"; anything more than this represents over modulation. The current specification for the British PPM has changed little since it came into use, and is closely defined to control:- - Attack Time (Step function and Transient response are separately defined). - Fall Time (Slow decay to scale zero point). - Mechanical pointer overshoot to FSD, on step function from zero. (Limited to 2% max) - Frequency response (Filtering is employed to restrict out of band signals) - Scale markings, PPMs use a very simple (near) linear, but clear scale. - Scale Law, and accuracy of conformance (PPMs have a special semi-logarithmic law) - Full wave signal rectification catches differences between + & - signal excursions. - Full scale deflection sensitivity error of the meter movement is limited to 1% max. - Modern PPM drive amplifiers have high input impedance and cause no line loading. - Even physical parameters such as scale and pointer length are defined. The actual precision meter movements for use in either VU or PPMs cost about the same, as they are both built to fine tolerances, but they have very different mechanical performance and appearance. The PPM however requires a special matched drive amplifier at additional cost, where as the VU meter is simply connected across the signal line. The advantages of the British PPM are considered by Broadcasters in the UK to far outweigh any additional cost, and mechanical meters are preferred in many applications. Experienced Practitioners find they are able to judge programme level accurately by simply appreciating the "angle" of the pointer, in much the same way as one tells the time on an analogue watch, without any of the digital fussiness of a bar graph display. Do not be deceived by this apparent simplicity, as mechanical PPMs easily respond to incremental changes of only 0.1dB with a silky smoothness. © Murraypro Sept 2005 .