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What is the minimum frequency response which should be represented by an audio codec? Well, what is the best analog or digital system and is it worthwhile to approach, match or exceed its specification?
Analog record players have an exceptionally good minimum frequency response. Technically, the minimum frequency response is determined by the duration of the recording. So, a 45 minute long-play record has a fundamental frequency of 1/2700Hz - which is about 0.0004Hz. Does this make any practical difference? Most definitely yes. People lampoon analog purists or stated that LP and CD are directly equivalent. Some of the lampooning is justisfied but the core argument is true. Most implementations of Compact Disc Audio don't achieve useful functionality.
Many Compact Disc players boldly state "1-Bit DAC Oversampling" or suchlike. The 1-Bit DAC is simplistically brilliant and variants are used in many contexts. However, oversampling is the best of a bad bodge. The optics of a Compact Disc are variable and, even with FEC in the form of Reed-Solomon encoding, about 1% of audio sectors don't get read. What happens in this case? Very early CD players would just output nothing. This technique was superceded with duplicate output. (Early Japanese CD players had an impressive amount of analog circuitry and no other facility to stretch audio.)
Eventually, manufacturers advanced to oversampling techniques. In the event that data cannot be obtained optically from disc, gaps are smoothed with a re-construction from available data. Unfortunately, there is a problem with this technique. Nothing below 43Hz can be re-constructed. 2KB audio sectors have 1024×16 bit samples and samples are played at exactly 44.1kHz. So, audio sectors are played at the rate of approximately 43Hz. However, any technique which continues audio waves has a fundamental frequency of 43Hz. Given that drop-outs occur with some correlation at a rate of 1%, this disrupts any reproduction of frequencies below 43Hz. For speech, this would be superior to GSM's baseline AMR which has 50Hz audio blocks. For music, this is a deal-breaker.
Remove the intermittant reading problems and the fundamental frequency is the length of the recording. So, a 16 bit linear PCM .WAV of 74 minutes has a fundamental frequency of approximately, 0.0002Hz. The same data burned as audio sectors on a Compact Disc only has a fundamental frequency of 43Hz. (I'll ignore the reverse of this process due to cooked sectors.)
So, it is trivial to retain low frequencies within a digital system. Just ensure that all data is present and correct. Furthermore, low frequencies require minimal storage and can be prioritized when streaming.
A related question is the minimum frequency response for a video codec. That's an equally revealing question.
(Score: 0) by Anonymous Coward on Monday July 10 2017, @01:28AM (1 child)
As opposed to a ~50ft 20Hz wavelength in air? Headphones (with multiple drivers) have advantages in terms of frequency response and directionality.
Yes obvious limitations considering they cannot possibly reproduce lower frequencies at the required SPL... while headphones can, *facepalm*
So it's that AC again, I'm checking back for replies. Not to beat on you, I'm actually (somewhat) interested in your project. You should check my claims and you should pay attention to working engineers and not "golden-ear" types or audiofools. Please keep posting ;)
(Score: 2) by cafebabe on Monday July 10 2017, @04:26PM
From the Audio Engineering Society [aes.org] Convention Paper 9178: A Hierarchical Approach To Archiving And Distribution [aes.org] (via here [aes.org]), page 4 of 16, and elsewhere, 50 foot of air attentuation isn't a significant problem below 10kHz. Indeed, size of sweet-spot is a more significant problem than air attentuation before reaching 10kHz. (I understand that Nimbus Records [wikipedia.org] performs spectral leveling to compensate for air attentuation but I prefer raw data.)
Regarding headphones, I use Philips SHP2600 with 32mm cones *specifically* for the purpose of editing speech. I don't require linear reproduction below 1kHz but a grumbled before buying them. In response to your message, I tried looking for information about headphones and I note that many brands give no useful information about cone size or frequency response. For example, Beats By Dre [beatsbydre.com] has headphones up to US$400 but fail to provide cone dimensions. The budget option opaquely specifies a 20Hz-20kHz range and the majority of products are those hateful earbud things.
For speech, I regard 32mm cones as compromising my professional integrity. For music, I regard 50mm to be similarly compromising my professional integrity. However, if it is possible to purchase 30 speakers at US$10 each (or significantly less) then it maybe worthwhile for experimenting. I expect the frequency response to be terrible but it might form a viable system if the frequency response is eqally terrible from all directions. If I'm wrong then the baseline specification will be raised.
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