Stereo Recording

History

The invention of stereo sound is accredited to Alan Blumlein in the 1930's. He effectively patented all of the ideas around stereo including mic techniques and cutting stereo records. Stereo recording then was facilitated by the introduction of the German invented magnetic tape recorders following World War II. Still, the majority of recording was mono until the 1960's, primarily because TV and Radio broadcasts were mono.

AEG engineers developed the first practical tape recorder, the Magnetophone K1, in 1934 Germany. It used magnetic tape invented by Fritz Pfleumer. The first demonstrations were at the Berlin Radio Show in 1935. A few German recorders were brought to the US after World War II, and ultimatetly led to the Ampex 200A in 1948.

The first recorders were one or two track, but recordings were still mono. You could, of course, use multiple recorders and bounce tracks back and forth between them. And, this style of recording was pioneered by Les Paul (yes, that Les Paul!), and ultimately resulted in his invention of the eight track recorder built for him by Ampex.

The Birth of Stereo: Although available experimentally as early as 1932, stereo records did not catch on until 1958. And still it would be several more years until it became wide-spread.

In the 1960's, The Beatles would use a four track tape recorder to record most of their early work. They would record on up to three of the tracks, and then bounce the three tracks down to one, which meant that the other tracks were available to add more parts. And everything was eventually mixed down and recorded to mono. It was only later that they started also doing a "stereo" mix. And since those were very early days for stereo, there were no standards for how a stereo mix should be done. So, they simply mixed to the two stereo tracks (left and right) in whatever manner they felt at the moment.

Note:

You can hear this in the stereo versions of those early Beatle records. For example, they might have drums all placed hard left! And, vocals hard right!

Note:

Stereo recording was enabled by the availability of tape recorders that were capable of recording multiple tracks, and the evolution of vinyl record technology to allow cutting stereo records.

Stereo recording techniques are a rich subject unto itself. Many techniques and methods are used to "spread" the sound into the stereo sound field. Sounds can be placed in the stereo field by varying the pan control, which is adjusts the left/right volume so that the sound can be heard as if it is appearing between the speakers. Or, it can initially recording as a stereo signal applied to two tracks. This can be accomplished with a variety of techniques, such as XY, ORTF, Blumlein, and MS.

The type of microphone is dependent on the stereo recording technique that is used. However, it is often a cardioid mic because we want to use the directionality to keep the sound independent as it is captured by each mic. The difference in sound captured by each mic is what helps to create the stereo image. Notable exceptions are MS (Mid/Side), which uses a figure-eight mic and a cardioid mic, and Blumlein which uses two figure-eight mics.

Note:

It is not only the pickup direction of the mic that we need to think of. We also need to think of the opposite direction, or rejection area, often called the "null". The null will allow you to record one sound source preferentially by pointing the cardioid mic at the desired source, and locating the undesired source in the null zone. You may still get some leakage, but it will be at a much reduced level.

Mic Setups for stereo recording are illustrated in the above diagrams.

XY uses two cardioid mics crossed at right angles to each other in the horzontal plane, with their capsules as close as possible to each other (see illustration). This will create a stereo image with limited filed width.

ORTF, devised around 1960 at the Office de Radiodiffusion Television Francaise (ORTF) at Radio France. This uses two cardioid mics 110 degrees with capsules pointed away from each other (see illustration). This will provide a broader stereo field at the expense of some phase issues, especially for sources close to the mic.

Blumlein is possibly the best of the stereo recording techniques. It is often considered the most natural, and candid, sounding. It uses two figure-eight mics placed at right angles with their capsules as close together as possible (see illustration). This gives a very natural sound field without phase issues. For Blumlein, neither mic is pointed directly at the source. Instead both mics are 45° angle to the source (see above illustration). So, there is an imaginary line pointing to the source.

Mid/Side uses a cardioid mic pointed at the sound source, with a figure-eight mic at right angles, and with the capsules as close together as possible (see illustration). This technique requires special setup in the DAW, or console. The cardioid mic is panned to the center. The figure-eight mic is split to two channels, with one channel phase flipped, and both channels panned hard left and right. The "magic" is that the stereo image can be adjusted based on how the mid and side channels are blended together. By increasing or decreasing the side volume level, you can make the sound field wider, or narrower.

Note:

For any of the stereo mic techniques that have the two mics positioned close to each other, you want the mic capsules as close as possible without touching. This is because you want the sound to arrive at both mic capsules at the same time. Any difference in the arrival time will affect the stereo image, especially at higher frequencies. The is referred to as time coherence. And, differences in the time of arrival can cause phasing issues.

Examples:

Omnidirectional mics pick up sound equally from all directions.

Examples include the Shure VP64A, and the Earthworks QTC40.

Cardioid mics, arguably the most common type, are represented by Shure SM57, SM58, SM7b, and ElectroVoice RE20.

SuperCardioid Mics tend to be used in more specialized situations, such as movie sets and live sound, because of their rejection of unwanted sound outside fo the pickup pattern. An example is the Rode NTG4 Shotgun mic which is used extensively in video recording.

And we must mention the many mics that have multiple polar paterns selectable by a switch on the mic body. Examples include AKG C414 and Neumann U87.