CD vs Vinyl Debate

Generated on request by Bing image Creator.

In order to reproduce sound faithfully, the Nyquist-Shannon sampling theorem says that a sound can be perfectly reconstructed from its samples if the sampling rate is at least twice the highest frequency component of the signal. That's not the fundamental frequency of the sound, it's the highest frequency component of the signal when its spectrum is viewed in the frequency domain. The timbre or difference in tone colour between a pure tone and e.g. the sound from an organ, both with the same pitch and loudness, is that the organ sound has a fundamental frequency and higher harmonics (multiples of the fundamental frequency). The combination of, and amplitude of harmonics is one factor that makes the timbre of a musical note from a piano different from that of e.g. a flute or violin. The discussion in the forum at the link suggests that CD recordings should be better than analogue recordings on vinyl, considering that sampling is done at 44 kHz (twice the 22 kHz bandwidth allowed for audio signals). However there are other factors involved, and another forum member suggests that compression of the signal is involved when the pressing master is made from the original master, and production CDs suffer in sound quality due to this. Compression is necessary to reduce dynamic range (the difference between the loudness of loud and soft sounds) so that recordings can be listened to on earbuds rather than a hi-fi system, the latter having greater dynamic range capacity. This compression of dynamic range is distinct from the compression of the size of files to reduce the storage space required on media. That compression is variable and produces either lower quality/smaller file size or higher quality/larger file size recordings, typically in the MP3 (for audio) or MPEG (for video) file format. This is similar to the way bitmapped images are compressed into a JPEG image file. Unlike MP£ files, sound tracks on a normal CD aren't compressed and the data is in a raw format that can be read by the electronics.

A debate on the subject here. 

18,000 Shipwrecks

There are an estimated 18,000 shipwrecks in Irish waters. To date, 480 have been surveyed and recorded in the INFOMAR shipwreck inventory using technologies such as single and multi-beam echo sounding, shallow seismic pinging and side scan sonar.

Mystery Aircraft and Beat Frequencies

Antonov An-12BK. Image courtesy Richard Vandervord via Wikimedia Commons. CC BY-SA 4.0 International.

Didn't manage to get the camera out in time, but I noticed the distinctive beat frequency of multi-prop engines. (Quite loud for the altitude). I checked with FlightRadar24 and it was an Antonov An-12BK cargo aircraft, flying at 22,000 feet on a flight from Porto to Prestwick and operated by Ukrainian Cavok Airlines. The AN-12BK is a four-engine turboprop military transport aircraft, dating from the Soviet era.

What are beat frequencies?

 

When two sound waves of different frequencies interact, the waves add together and the variation in amplitude results in a tremolo effect; a periodic rise and fall in volume. (Basically the peaks or troughs combine and reinforce or cancel each other out. ) The phenomenon is sometimes noticed when two musical instruments with different pitches are played together. Beat frequencies also occur with radio waves, electrical signals and coherent light waves.

The animated GIF below show a red and green sound wave, initially at the same frequency and in phase. The two waves add together to form a blue sine wave that's greater in amplitude. As the frequency of the green wave slowly changes, the resultant blue waveform changes in amplitude, modulated by a beat frequency. See link also in the comments to a YouTube demo, showing the phenomenon with two tuning forks.

Anyone who's doing their Junior Cert will probably be able to do the maths and find the resultant of two sine waves of different frequencies added together, requiring the trigonometric identities at the end of the maths tables to do the calculations (Are these still available from the Government Publications Sales Office?)

 

More information on beat frequencies here:
https://en.wikipedia.org/wiki/Beat_(acoustics)

 

Beat frequency. The blue waveform is the sum of the red and green waves. Animated GIF courtesy Adjwilley via Wikimedia Commons. CC BY-SA 3.0 Unported.