March 21, 2023



Know Audio: Stereo | Hackaday

5 min read

In our occasional sequence charting audio and Hello-Fi know-how we now have handed at a technical stage the primary elements of a house audio set-up. In our final outing after we checked out cabling we left you with a promise of overlaying instrumentation, however now it’s time as a substitute for a brief digression into one other subject: stereo. It’s a phrase so tied-in with Hello-Fi that “a stereo” is an alternate phrase for nearly any music system, however what does it actually imply? What makes a stereo recording, and the way does it arrive at your ears?

From West London Trains, To 3D Audio

The motive force of this Nice Western Railway practice had no concept that he was making audio historical past.

As most of you’ll know, a mono recording makes use of a single microphone and a single channel whereas a stereo one makes use of two microphones recording concurrently a left and proper channel. These are then performed again by way of a pair of audio system, and the result’s a way of spatial area for the listener. Devices seem to come back from their relative positions when recorded, and the sense of being within the efficiency is enhanced.

Stereo recording as we all know it was first perfected as one of many many innovations credited to Alan Blumlein, then working for EMI in London. Now we have certainly one of his stereo demonstration movies in “Trains at Hayes“, filmed from the EMI laboratories overlooking the Nice Western Railway, and that includes a sequence of steam-hauled trains crossing the sector of view with a corresponding stereo sound area. His work laid down the basics of stereo recording, together with microphone configurations and what would turn into the usual for stereo audio recording on disk with the channels on the other sides of a forty five diploma groove.

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Extra Than Simply A Combine

On the face of it, a stereo recording delivers solely left-right positional info, and thus will be created from a sequence of mono tracks by adjusting their place within the stereo combine. For instance, a band might have the bass guitar positioned largely within the left channel and the lead guitar in the precise, with the drummer and vocalist equally in each channels to put them within the centre. Take heed to “Yellow Submarine” totally panned.

Within the trendy period, a real stereo recording picks up rather more than the relative intensities of various sounds, it data the advanced internet of timing and section variations within the sound together with these within the background noise. Thus a stereo recording created from a mixture of mono recordings can finest be known as pseudo-stereo as a result of it lacks that section info which supplies the stereo recording a lot depth.

In an analogue tape deck or a vinyl document the stereo channels are recorded individually as tape tracks or reverse sides of the groove. In the meantime in digital audio techniques the left and proper channels arrive as bitstreams both from an interleaved i2S supply resembling a CD participant, or from a decompression algorithm in for instance an MP3 participant or web streaming software program. There may be nevertheless one place wherein you’ll nonetheless generally encounter a stereo supply that makes use of an analogue encoding system, specifically FM radio. This contains a system wherein the primary broadcast is in mono, however the stereo info is individually encoded on the transmission at an inaudible frequency for decoding within the receiver.

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The Final Bastion Of Analogue Stereo

All the information in an FM stereo broadcast, represented as a spectrum. Aboce the audio on the left is a 19kHz pilot tone, then the stereo difference signal modulated on a 38kHz carrier.
All the data in an FM stereo broadcast, represented as a spectrum. Aboce the audio on the left is a 19kHz pilot tone, then the stereo distinction sign modulated on a 38kHz provider. Wollschaf (CC BY-SA 3.0).

For the needs of FM broadcasting, the left and proper channels are mixed into sum, or L+R that represents the mono part, and a distinction between the channels, or L-R. These are shortly generated utilizing a simple op-amp circuit. The L+R is modulated because the audio you hear on a mono receiver, whereas the L-R is encoded individually as a 38 kHz double sideband sign out of the audio vary. A 19 khz pilot tone is added between the audio band and the stereo info, and within the demodulator that is used with a frequency doubler to offer a 38 kHz tone to demodulate the L-R distinction part. That is then added and subtracted from the mono sign to rebuild the left and proper channels. This method originated in a Zenith proposal from the early Nineteen Sixties, and stays in use worldwide at this time.

There are an enormous number of sign processing methods to boost the stereo expertise or produce stereo results. Only one instance is Dolby Atmos, and you’ll hear them at work in lots of pc video games and movies. All this digital magic just isn’t the one technique to play with stereo although, a favorite Seventies venture was a stereo expander. These units labored by subtracting a small quantity of the left from the precise and vice versa, having the impact of lowering the proportion of L+R and growing the proportion of L-R within the output. In as of late of low cost DSP, some skilled audio is shifting towards the same Mid/Aspect encoding.

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Binaural, It’s All In The Ears

A dummy head studio microphone for binaural recording. EJ Posselius, CC BY-SA 2.0
A dummy head studio microphone for binaural recording. EJ Posselius, CC BY-SA 2.0.

There’s one remaining piece to the stereo puzzle, which you might need seen if you happen to’ve ever encountered a recording described as “binaural”.  A stereo recording is mostly meant to be performed to the listener utilizing a pair of audio system, in a great state of affairs the place the audio system and the listeners head type the factors of an equilateral triangle. On this method every ear hears one thing of each left and proper, with whichever aspect the ear is closest to being the dominant of the 2.

In a binaural recording every channel is meant just for one ear, so it’s designed to be listened to on a pair of headphones. Binaural recordings are sometimes made with microphones designed to duplicate the acoustic traits of a human head with microphones positioned in its ear canals, with the intention of manufacturing the phantasm of a real-world directional audio area. YouTube is stuffed with examples, and we’ve picked this traversal of central London to offer you an concept.

So whether or not you’re an audio fanatic or are glad with the most affordable of dollar-store audio system, we hope you’ve loved our high-fidelity journey into the world of two-channel audio. Keep tuned for extra, as we’ll return with one other on this Know Audio sequence.

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