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High-resolution volume-unit meter simplifies CD recording

-June 21, 2001

Digitally recorded music on CDs offers superior quality to that recorded on vinyl records or tape, but most prerecorded CDs have an annoying characteristic: The average volume levels of the recorded signal can vary by as much as 14 dB from disk to disk. Significant variances, such as 4 dB, can occur from track to track on a single CD. This variation can be a problem when recording your own CDs because you can end up with large variations in loudness between songs.

CD digital-recording decks typically have peak-reading-only volume-level meters. This feature is adequate to prevent clipping but does a poor job of reading the average volume, or loudness, level. If you set the recording levels just below the peak clip level, average volume variances of 6 to 8 dB can result due to varying levels of dynamic-range compression in recording the original material.

A high-resolution, average- (not peak-) reading volume-unit meter produces an accurate reading of loudness (Figure 1). The meter connects to the line outputs from the recording deck to monitor signal level. IC1A sums the left and right channels and adjusts gain. This adjustment allows you to calibrate the 0-dB LED, D1, to the signal level of your CD recorder.

IC1B is a precision rectifier, which sends the positive portion of the signal to the input of IC2, an LM3914 dot/bar-display IC. R1, R2, and C1 filter the signal to the input of IC2 to provide an averaging effect. R1 adjusts the rise time of the signal, which increases or decreases the meter's ability to track a fast rising signal. Increasing R1 slows rise time, making the meter more average-reading than peak-reading.

IC2 contains all the necessary circuitry to drive a 10-LED string as well as an internal reference. R3 and R4 bias up the bottom of the resistor string to 0.4V above the ground pin, which effectively reduces the total decibel range of the meter and increases the resolution and accuracy of each LED step.

Another feature of IC2 is that the resistors connected to Pin 7 externally program the LED current. The resistor values in Figure 1 result in an LED current of approximately 8 mA. To conserve power, the circuit leaves the bar/dot-select pin, Pin 9, open to select 'dot' mode. IC2 has some built-in overlap, so that at least three LEDs are on at any time during typical operation of the meter, which eliminates flicker and makes the meter easier to view.

You use this volume-unit meter with the peak-reading meter in the recording deck. You should initially set up the recording levels by finding the loudest portion of the music and adjusting the record level of the deck to approximately 4 to 5 dB below the peak clip level the meter shows on the CD deck. The CD- deck makers advise against any clipping because it causes distortion. Backing off by 4 to 5 dB from the clip level allows some headroom in case later-recorded tracks have higher dynamic range (the ratio of peak signal level to average signal level). You should then set the gain-adjust potentiometer, R5, so that the 0-dB LED lights on the loudest sound. Adjust subsequent track-record levels to this same level, based on the average loudness, to ensure that the deck's recording meter does not exceed the clip level.

Adjust R1, the rise-time adjust, so that the meter will respond to average level changes but miss fast peaks. This control is user-adjustable, and the optimum setting depends on the music type.

The meter's bandwidth is approximately 250 Hz to 2 kHz; the upper limit depends on R1's setting. This bandwidth selects the range of sound that most reflects perceived loudness based on the ear's frequency response. Lower bass signals typically have large signal levels in absolute magnitude, but the ear hears these signals less due to the Fletcher-Munson effect. If they register on the meter display, the bass-frequency signals would indicate higher loudness than you actually hear.

The ear also perceives very-high-frequency sounds less loudly than absolute magnitude would suggest, although the effect is not as pronounced as the low-frequency roll-off. So, to get an accurate approximation of loudness, the meter measures the middle range where the ear is most sensitive.

The battery current was measured and found to be 22 mA while operating, which means the expected battery life of a 9V alkaline battery should be at least 20 hours. Because the meter sets only record levels, you can switch it off during recording to conserve the battery.

The circuit connections to the external stereo system are in the line coming from the line outputs of the CD-recording deck and going to the input of the receiver/amp. The meter effectively connects in parallel with the input of the receiver/amp. For this reason the volume-unit meter's input impedance is very high to prevent loading the line output.


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