Post AWCGtBLoo44eeVF7tQ by lore@berserker.town
(DIR) More posts by lore@berserker.town
(DIR) Post #AWCE9xXP7JB8s7yEIS by lore@berserker.town
2023-05-30T23:45:11Z
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the typical format that you see for processing digital audio is called PCM - pulse code modulation.it's not a very helpful or descriptive name though.what it refers to is storing audio waveforms as a series of instantaneous sound pressure measurements called samples, typically at rates of 48000 Hz (samples per second) or above.the samples can be taken at varying levels of precision. 16-bit PCM might use a value of +32767 for the maximum positive sound pressure, 0 for no pressure and -32767 for the maximum negative pressure.a tone would consist of a series of such samples that trace out a repeating sound waveform if you plot them on a graph.actually measuring or reproducing a sample to degree of precision electrically is difficult though. using a classic converter design, you start running into difficulties above 8 bits of precision - insufficient for high-fidelity audio reproduction.sometime in the late 80s, a new method of sampling was devised. around that time, manufacturers began to switch to what is sometimes called 1-bit converters. the technically correct name is sigma-delta converter.instead of trying to take absolute sample values, what they do is track the relative change since the last sample, but in a very simple way: 1 means a step up, 0 means a step down. and they do this at 128-256 times the advertised PCM sample rate.to produce a PCM sample from this bit stream, they add these samples up digitally to produce an average.SACD - the failed successor to the CD - ditched PCM altogether and uses this bitstream format on disc instead.PCM hasn't been the "native" format of digital sound for at least 30 years now. but it's what everyone is used to, and it's easier to process than the raw sigma-delta bitstream, so it has stuck around.
(DIR) Post #AWCF7uD2cYSzvM1x6e by andersgo@oslo.town
2023-05-30T23:55:59Z
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@lore @thor I vaguely remember CD players marketed with 1 bit converter
(DIR) Post #AWCFzdR3CbtXXPCftw by lore@berserker.town
2023-05-31T00:05:44Z
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@andersgo they're much easier to manufacture. the classic design with resistor ladders is still used for applications where speed is more important than precision, or where absolute measurements are needed. sigma-delta converters aren't so useful for recording or reproducing slow-moving signals that settle at specific levels.
(DIR) Post #AWCGtBLoo44eeVF7tQ by lore@berserker.town
2023-05-31T00:15:45Z
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fun fact: an ideal sigma-delta converter (a theoretical one with an infinite dual power power supply) can't ever clip. no matter how positive or negative the voltage on the input is, it doesn't matter, because it's only measuring relative change over time. in most practical applications, the voltage rails are probably 3.3V apart, so the input would clip at +/- 1.65V. that's still higher than your typical line level signal though. it's the PCM values that clip, because they have a limited range.
(DIR) Post #AWCHa1nIDMCRSqWZSi by lore@berserker.town
2023-05-31T00:23:31Z
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there's no theoretical reason you couldn't design a sigma-delta converter that used floating point values for the PCM samples. then you'd have 0 dBFS at +/- 1.0 and as much headroom above that as the power rails to the chip would allow. at the top of that range, you could use diodes to soft clip it before it hits the converter. with current converters, going above 0 dB is a mortal sin because of the nasty distortion it produces. but it doesn't really have to be that way.
(DIR) Post #AWCJLi5qrdUCDZVPSy by Scrawly@noagendasocial.com
2023-05-31T00:43:21Z
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@lore TIL — thanks!
(DIR) Post #AWCdVe7oTHp1tCREfI by grant_h@mastodon.social
2023-05-31T04:29:13Z
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@lore next Klang project? Non peaking adc would be cool!
(DIR) Post #AWD56VgpUfYJClcTMu by andersgo@oslo.town
2023-05-31T09:38:21Z
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@lore Bitstream was the marketing term. I remember I had this beast in the early ´90s