A young engineering genius used a computer to design a filter and the result was the greatest FM tuner ever built
Turn On, Tune In: In the years before Internet radio and streaming music services, a good FM tuner was a must for audiophiles. None was better than the McIntosh MR 78, which cost an eye-watering US $1,699 when it was introduced in 1972.Photo: HiFiCollector.com
McIntosh’s MR 78 was a trailblazer, the first AM/FM tuner precise enough to tune in the weaker of two radio signals broadcast on very close frequencies.
That seems like a narrow qualifier, but it was a remarkable technical advance. Why was it such a big deal? Like many of the other items in our Consumer Electronics Hall of Fame, including the Fuzzbuster, the Walkman, and the Cobra 138XLR CB radio, the McIntosh MR 78 satisfied a need that was at once technological and cultural.
When broadcast radio began as a business in the 1920s, all radio used amplitude modulation (AM). Frequency modulation (FM) was invented by Edwin H. Armstrong in the 1930s, but FM radio stations did not begin appearing until the 1940s. Music on FM sounds better, but that didn’t make that much of a difference until the 1960s, when mono recording gave way to stereo. FM could support stereo starting in 1961, but AM wouldn’t until the 1970s. Music programming finally began shifting to FM stations.
Top 40 stations on the AM dial still dominated the ratings. But as the 1960s progressed, musicians were writing songs that got progressively more unconventional or longer, which disqualified them from AM’s regimented Top 40 format. A growing number of FM stations started popping up to play this increasingly popular music.
Listeners had no reason to tune in to the Top 40 station from the next town over because every Top 40 station was playing almost the same 40 songs as every other. But suppose listeners wanted to hear something by an artist that didn’t fit that format—at least 99 percent of all music released, surely. Then they had to search for a station that would play that artist. So for listeners with adventurous taste in music, searching for distant stations—spinning the dial—became an imperative in those pre-Internet days.
A signal that is strong, because it is higher-powered, physically closer, or both, will of course drown out a weaker signal on the same channel. From the 1920s through the 1950s in the United States that interference was almost never a problem, because radio stations had always had generous space in terms of both listening area and assigned frequency.
But by the early 1970s, the airwaves were getting crowded and interference was becoming more common. The number of stations in the United States had jumped from 5,134 in 1963 to 6,530 in 1970, and the likelihood that two stations might be near each other both physically and on the radio dial had gone up. People were spending more time listening to the radio, and many of them were trying to tune in stations with signals that were weaker, either because they were far away or were transmitting at lower power (such as many college stations).
And contemporary tuners weren’t up to the task. These included McIntosh’s MR 71, the Fisher FM-1000, and the Marantz 10B. “They all had the same wide-bandwidth signal issue,” says Ron Cornelius, a salesman who sold many of those systems at the time (he was hired by McIntosh directly in 1993). Radio stations are allowed to broadcast in 0.2-megahertz-wide bands centered on their assigned frequencies, but even the best tuners at the time were imprecise and tuned in bands wider than that—the “wide-bandwidth signal issue” Cornelius refers to.
Because radio stations were at first well spaced, they never had any pressing need to be scrupulous about remaining strictly within their assigned 0.2 MHz. In fact, the imprecision of tuners was an incentive for some stations to transmit signals that exceeded the edges of their assigned bands; it was occasionally a deliberate attempt to match the wider bands that tuners used.
When the airwaves began to get crowded, however, imprecise tuners began to be problematic. They were inherently unable to filter out adjacent signals, a problem exacerbated by those stations slopping out past their allotted 0.2 MHz.
Richard Modafferi was a senior engineer at McIntosh Laboratory from 1968 to 1974. His first stab at a new tuner that could do better was the MR 77. That model, introduced in 1970, was acclaimed among audiophiles—and it was the subject of an article by Modafferi in the November 1970, issue of IEEE Transactions on Broadcast and Television Receivers. And then Modafferi outdid himself with the MR 78 tuner, which came out in 1972.
The MR 78’s distinction was a new intermediate frequency filter, the culmination of a design that Modafferi had begun working on [PDF] as a graduate student at the Newark College of Engineering (now part of the New Jersey Institute of Technology) in the mid-1960s. He dubbed it the Rimo filter (a concatenation of the first two letters of his first and last name). In a superheterodyne radio receiver, which all FM receivers are, the incoming signal is mixed with that of a local oscillator to produce an intermediate frequency (IF) that is lower than that of the carrier frequency of the received signal. There are several reasons for doing this. A really big one is that the lower value of this IF makes it easier to filter out nearby sources of interference. Basically, Modafferi’s Rimo took a leap beyond existing IF filters by applying a computer to fine-tune the filter’s frequency response, and also by minimizing “intermodulation distortion,” which plagued IF filters at the time.
Regarding the Rimo, “It is the correct width to let just one FM station through,” McIntosh states in the owner’s manual for the MR 78 [PDF]. “The excellent selectivity of the MR 78 (210 kHz wide at 60dB down) permits tuning stations that are impossible to receive on ordinary tuners.” Touting the great “mathematical complexity” of the new filter, the manual also asserts that “an IBM 1130 high speed computer spent eighteen minutes on the mathematics for the design of the IF filter. It would have taken an engineer, working twenty-four hours a day, seven days a week, and working error-free three-hundred years to perform the same mathematical calculations.”
One of the more eye-popping disparities between the Rimo filter’s performance and the performance of competing products was in delay distortion. “All other IF filters have delay distortion, as much as 100%,” the owner’s manual rhapsodized. “The MR 78 filter has less than 1.0% delay distortion from antenna input to discriminator output,” it added.
In addition, the company built a new FM detector, also designed by Modafferi, to work with the new filter. The new filter, the new FM detector, and other improvements added up to the MR 78 having the narrowest IF bandwidth ever achieved in a stereo tuner, the company claimed.
Handsome Beast: The MR 78 weighed in at 12 kilograms (27 pounds) and was available with an optional walnut-veneer cabinet.Photo: McIntosh Laboratory
People paid for that level of performance. The original list price for the MR 78 was US $1,699, which translates to a whopping $10,000 in today’s dollars. Even then, the price was roughly 10 times as much as that of the average tuner. Today, when an MR 78 pops up on eBay in decent shape, it generally fetches more than $1,000.
The high price notwithstanding, audio salesman Cornelius says the MR 78 “was very popular at the time as it was a demonstrable solution to a common problem. With a large FM antenna, you could receive stations from one to many hundreds of miles away.”
He adds that the MR 78 solidified McIntosh’s position in the highest echelon of tuner manufacturers. It had an immediate effect on the rest of the market too; it forced all the competitors to scramble to match it. “They all had to develop ‘super tuners,’ usually FM-only, to compete,” he recalls.
Editor’s note: A fascinating brief history of the Rimo filter was published as a letter, from Modafferi himself, to the editor of Audio magazine in May 1977. The complete issue is available online in PDF format.