Dorrough 280 C Analog Loudness Meter

The Dorrough 280 C Analog Loudness Meter is a straight line, horizontal two-channel loudness meter with 20 dB of headroom.

For nearly 60 years the world has used two different standards for audio monitoring. In the United States, the VU meter, introduced in 1937, was the standard for level monitoring. In Europe, the PPM (Program Peak Meter) was the standard. The VU meter is a quasi-average reading device. Basically a voltmeter calibrated in power with ballistics chosen to represent early radio and film program material, it was never intended to indicate the peak excursions found in today’s program material.

The PPM addressed the limitations of the VU meter, but because it displays and holds only the peak level of the waveform, the integration time of the quasi-average program level is almost completely ignored.

Most contemporary studios have tried to solve their metering problems by using both VU and PPM meters on the same source in an attempt to get the maximum usable level out of a piece of program material. This has led to cramped and overly complex metering bridges and panels, and does not overcome the limitations of these systems. Even the switchable bar graph meters offered as a solution by some console manufacturers suffer from the same ballistic limitations.

In 1980, after years of hands-on experience and frustration with both conventional VU and PPM metering, Dorrough research led to the introduction of the Dorrough Model 40 Loudness Meter. By providing an easy to understand display showing the relationship between integration of RMS and peak level display, the model 40 was an overnight success. Today the model 40 is regarded as the standard in audio monitoring and is used throughout recording and broadcast facilities around the world.

The Dorrough Loudness Meter gives you a much more accurate picture of the actual energy content of audio program material than conventional metering. By simultaneously displaying two ballistics, the Dorrough Loudness Meter shows you the relationship between average and peak. You need only adjust the program level until either the peak or persistence reference is reached. This will result in the maximum level, regardless of program content. Material with or without compression can easily be matched for the same listening level.

Into the Digital Domain

The Audio Engineering Society (AES), together with the European Broadcasting Union (EBU), have established a digital audio interface standard known as the AES/EBU interface. This standard provides robust formats for the exchange of digital audio information between professional audio devices, yet also provides flexibility for specialized applications. The International Electrotechnical Commission (IEC) has also endorsed a format based on the AES/EBU standard for consumer digital audio devices. Briefly, the AES/EBU format standard defines how two channels of audio information are periodically sampled, uniformly quantized and transmitted over a twisted wire pair. Left and Right audio channels are multiplexed, and the channel is self-clocking and self-synchronizing. The format is independent of sampling frequency and can be used with any sampling frequency including 32 kHz, 44.1 kHz, and 48 kHz, which are those recommended by AES for PCM applications.

The Dorrough 280 and 380

The Dorrough model 280 and 380 Digital Loudness Monitors (DLMs) provide the same basic functionality as the classic 40 series analog loudness meter except that they accept a digital data signal and display two channels.

The model 280 is designed for horizontal mounting while the model 380 is designed for vertical mounting. Both models are offered with two scale ranges, a standard 40 dB range (-D suffix), and an expanded 60 dB range (-E suffix). Figure 1-1 illustrates the model 280 with both scale options, while Figure 1-2 shows the model 380 with both scales.

Two Channel Display

As defined by the AES/EBU Digital Audio specification, two audio channels (A and B) are encoded into the serial datastream. On the model 280, Channel A (corresponds to Left), is displayed on the top scale, while Channel B (Right) is displayed on the lower scale. On the model 380, Channel A is displayed on the left scale, and B on the right. When multiple meters are installed in a console, channel designation can be made on the meter bezel (if used), or the meter bridge front panel.

Overrange Indication

Overrange indication occurs when the input signal reaches a point one count over maximum allowed input level. When this happens, the top three LEDs (0, -1, and -2) all change color to red while tracking the program level. Overrange indication is set at the factory to occur at the point one count over the maximum signal level (the maximum level is the point at which the signal becomes all binary 1’s). This point can alternately be set to 4 counts over the maximum signal level by moving a jumper.

Peak Hold Functions

Both the model 280 and 380 provide three peak hold functions: 3-second peak hold (normal), indefinite peak hold, and no peak hold. These functions are selected through a three-position toggle switch (supplied), or if only a single function is desired, jumper selected on the DLM main circuit board. Normal operation (peak hold for 3-seconds) is provided at the center position of the toggle switch, or through jumpering as described in Section 4 of this book. In this mode, the highest peak is held for 3-seconds before being reset.

Indefinite peak hold retains the highest peak that has occurred for an indefinite period while constantly resampling. This function is particularly useful for unattended operation where the engineer wishes to record the highest audio peak occurring during a mastering operation, or at a remote transmitter site, etc. The peak is cleared by switching the toggle switch to normal, then back to indefinite peak hold.

Peak hold operation may also be completely defeated. In this mode, peaks are displayed in real time.

Compatibility

Conforms to all formats defined within the AES/EBU Digital Audio specification, including IEC 958, S/PDIF, and EIAJ CP-340 interface standards. These standards include professional 24-bit and consumer 16-bit formats. Sampling frequencies include 32 kHz, 44.1 kHz, and 48 kHz.

Signal Interface

RS-422 levels over 75-ohm single-ended and 110-ohm balanced inputs. Transformer isolated (as dictated by the EBU specification) with series DC blocking capacitor on input and output. A buffered feed-through is also provided. These signals are not polarized.