Building the L-R Test Set
Introduction
In the process of broadcast matrix encoding and decoding, balancing
amplitudes between the two channels of a stereo audio path is very
necessary. It is necessary for the maintenance of proper headroom,
dynamic range and, of course, separation.
From time to time, repair and maintenance must be performed on
equipment. Returning the entire chain to a balanced condition when
placing the equipment back in service, is very desirable. Most
broadcast engineers have balanced a chain at the stereo generator using
the L-R method. But why not balance the output of each piece of
equipment throughout the studio and eliminate the trip to the
transmitter? It could mean eliminating late nights, and for those who
don't go off the air, it may be the only option.
While recording studios do not face the same problems as the broadcaster,
much of what is recorded in a studio is broadcast. This technique,
therefore, is also applicable to the maintenance of a final stereo
path in a recording facility.
Review
Measuring the amplitudes of the two channels using side by side
VU or PPM meters at best yields a balance to within a dB or two.
This is not close enough for today's standards. Vastly improved
resolution is possible by measuring the difference between the
two identical mono signals by performing an L-R
null at each point along the chain. The adjustments are made using
mono program material in the signal chain. The degree of
balance achieved will allow you to be confident that the listener's
stereo hasn't been degraded by the process.
Fig 1.- L-R Test Set
The test set uses well-trimmed difference amplifier circuits.
The noise floor is the practical limit to the depth of null that
can be achieved with identical signals. This property allows signal
amplitudes to be matched to exceedingly small differences, 1 part
in 10 to the 5th, or better. Additionally this type of measurement
will allow an engineer to determine which pieces of equipment
have differing inter channel time delays or phase shift, the other
property that upsets channel separation.
Construction
The L-R test set is easily constructed with three Benchmark Media
DIA-1 differential input modules and some form of wide band AC
voltmeter. The test set is shown in Figure 1. Two of the differential
inputs are used to interface to balanced lines while the third is the
L-R amplifier. We suggest building the DIA-1s into a Benchmark IFA box,
and powering the set from a Benchmark PS-1 ±16 volt wall mount
power supply. The drawings presented here include the now obsolete
HPA-1 headphone amplifier. The new HPA-2 will perform
significantly better than the older HPA-1 and is highly recommended.
Once you have the differential amplifiers installed in a chassis and
wired you should power up to verify that everything is working and
wiring shorts haven't crept into the project.
Even though the DIAs were precisely adjusted at the factory, the
one used to measure L-R null will have to be re-adjusted. This is
because the gains of the two input DIAs, while extremely close,
are not perfectly identical. A re-trim will adjust for
that difference. You may want to include a headphone amplifier to listen
to the nulling, as well. The following is the recommended CMR adjustment
setup to perform the L-R circuit readjustment.
Setting up the L-R Circuit
The common mode rejection trims are part of a passive bridge.
Adjustment of the trims is an adjustment of the balance of this bridge.
In this case we are adjusting for the slightly different gains of the
input DIA-1s.
Fig 2 - L-R Circuit Adjustment
Feed a balanced signal of +20 dBu into both inputs of the test set.
Send a 2 kHz signal to the input and adjust the resistive portion of the
third DIA-1s bridge for a minimum audio output. It is very helpful to
watch the audio signal from the meter output on an oscilloscope, which
in turn should be synchronized to the signal source. This will allow you
to quickly see the phase-amplitude nulls as they take place. This is one
place where a digital voltmeter is almost impossible to use. The easiest
meter type to use is a logarithmic type. DBX and Valley People have
manufactured these items in the past, and may be available on the used
market. If you can't put your hands on one of these, a conventional AC
VTVM will work. Once a minimum resistive trim has been achieved, null
the capacitive trim.
Two or three iterations between these controls should be sufficient
to achieve the best broadband null possible. A null of better than 100
dB at 200 Hz, and better than 75 dB at 20 kHz, is usually achievable.
Unfortunately, it is almost impossible to maintain this precise a null
over much of a temperature range. A degradation of up to 10 dB may be
expected.
Using the Test Set
Using the test set is straight forward. Place the audio console into
mono mode. If this is not available, you will have to add this
capability to the console. Using program material move through the chain
and make sure that the gains of the left channels are all at unity by
using the L-R test set to compare the input level with the output level.
You will not be able to achieve a perfect null in this adjustment since
there will be time delay through the equipment. However the best null
will set the amplitudes at a precision unity gain. Next balance the
amplitudes between the two channels of each piece of equipment, making
sure to only adjust the right channel gains. Finally adjust the L-R null
at the stereo generator. This completes the chain set-up.
Fig 3. - Chain Set-Up Using L-R Test Set
As implied above, it is possible to measure the differential phase
shift between channels of equipment by measuring the depth of null and
doing a small computation. Additionally it is also possible to add phase
equalization to the audio chain and correct for signal delay differences
between the channels. We may examine this in a future app-note.
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