Specifications

15 YEARS OF BASIC CIRCUIT AND CONCEPT RESEARCH
HAS DEVELOPED THE TECHNOLOGY
NECESSARY FOR A TRULY LOW DISTORTION
FM TUNER WITH VARIABLE SELECTIVITY . . .
THE MCINTOSH MR 78!
Mclntosh research is a continuous program of ex-
ploration for technology that permits performance
improvement and greater value for you. This
unrestricted research program developed the
technology necessary for the realization of these
design concepts for the MR 78.
THE DISTORTIONLESS IF FILTER
Ever since the beginning of FM, research
engineers have realized that constant delay IF filters
equivalent to linear-phase were necessary for low
distortion reception. Crude approximations to cons-
tant delay have always been used in FM tuners - with
disappointing results. So-called "Butterworth" or
"Legendre" filters offer only a fair compromise with
respect to delay error. Crystal and ceramic filters,
usually based on the "Chebychev" model, work fair-
ly well and give good selectivity, but none of these
are constant delay linear-phase, filters. Commercial
tuners using these filters can show 5% stereo IM
distortion at 100 % modulation. The filters used in
most of todays tuners can have delay errors up to
100%! The IF filter in the MR 78 has a delay error of
less than 1% from antenna input to discriminator
output! In its useful bandpass, it is a true
mathematical approximation to linear-phase. It is
the world's finest selective, linear-phase, minimum-
phase shift filter.
A FORTRAN computer program using an
algorithm that took six years to develop was used in
its design. The mathematical complexity of the filter
design procedure is almost beyond belief. Using a
process called "numerical integration in the com-
plex S-plane," a high speed IBM computer was used
on the design of the IF filter. A human engineer,
working twenty-four hours a day and seven days a
week, would have taken 300 years to finish this work
. . . assuming he made no mistakes!
VARIABLE SELECTIVITY
The MR 78 has variable selectivity. In SUPER-
NARROW the IF bandwidth is 210 kHz wide at 60 dB
down and permits tuning stations that are impossi-
ble on ordinary tuners. The great number of stations
crowding the FM band requires a tuner with variable
selectivity.
In the NORMAL position, a very low distortion
8-pole filter is used in the IF circuit.
The NARROW position adds a sharp 8-pole filter
to the NORMAL IF filter to yield a low distortion (less
than 0.2%), highly selective 16-pole composite IF
filter.
SUPER-NARROW position adds a 4-pole 4-zero
crystal filter to the two other IF filters. SUPER NAR-
ROW permits receiving distant stations which are on
channels adjacent to local stations. With an ade-
quate FM antenna there are usable signals never
heard before with ordinary FM tuners.
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