Maths question- bits and kHz

[gurjitsidhu]

Hi

I was thinking about the term 128, 256 and 320kbps and it got me thinking about what this means in terms of bit rate and frequency.

Most people say 320kbps is a very good cd quality however when i done the maths it didnt quite add up...

CD quality is 16bit 48khz i believe which equates to 48,000 * 16 = 768kbps. 320 kbps actually equates to 16bit 20khz if we do the maths so the quality aint really CD quality

24 bit 96khz = 2,304kbps

24 bit 192khz = 4,608kbps

is this correct or have i made a mistake?

 

[andyjm]

Hi

I was thinking about the term 128, 256 and 320kbps and it got me thinking about what this means in terms of bit rate and frequency.

Most people say 320kbps is a very good cd quality however when i done the maths it didnt quite add up...

CD quality is 16bit 48khz i believe which equates to 48,000 * 16 = 768kbps. 320 kbps actually equates to 16bit 20khz if we do the maths so the quality aint really CD quality

24 bit 96khz = 2,304kbps

24 bit 192khz = 4,608kbps

is this correct or have i made a mistake?

I think you are a little adrift.

CD sample rate is 44.1KHz, not 48KHz, and there are two channels for stereo, so CD data rate is 44,100 * 16 * 2 = 1.411 MB/s.

In reality it is slightly higher, as the data stream contains framing and control information as well.

CD data is simple PCM, where as MP3 uses a compression algorithm to remove data which (apparently) the ear is unable to detect. The compression algorithm is lossy (unlike FLAC) so you never get the original data back, but it is close enough for the ear to be fooled.

So while PCM would take 1.411 MB/s to record stereo sound, MP3 can do the same at 320KB/s or approximately 4:1 compression.

 

[eggontoast]

128, 256, 320kbps refers to compressed music bitrates, none are CD quality although there are a few people that can't hear the difference with higher bitrates.

CD's sampling frequency is 44.1kHz not 48kHz.

 

[ID.]

128, 256, 320kbps refers to compressed music bitrates, none are CD quality although there are a few people that can't hear the difference with higher bitrates.

This. maybe your confusion comes from many people saying the difference between 320kbps is and CD is inaudible.

 

[andyjm]

128, 256, 320kbps refers to compressed music bitrates, none are CD quality although there are a few people that can't hear the difference with higher bitrates.

CD's sampling frequency is 44.1kHz not 48kHz.

If you search around the net for real ABX tests, it would seem that most people can't tell the difference between 320Kb/s MP3 and 1.411 Mb/s CD.

 

[JohnKK]

It doesnt have anything to do with tests because we dont know what king of system do people use.

Previously I had budget AVR and budget bookshelfs: 256, 320, FLAC with 900kbps -all sound the same.

Now I have move up with gear to stereo amp and standalone DAC and I can hear huge difference btw 320 and 900 flac. The difference is totaly obvious.

So my conclusion - it is all to system quality and what you are used to.

To the original question ;

I am not that much i scientific part of digital mucis, but you can not multiplay (spelling?) bits and Hz and get kbits?!

to me this doesnt make any sense.

 

[Overdose]

128, 256, 320kbps refers to compressed music bitrates, none are CD quality although there are a few people that can't hear the difference with higher bitrates.

CD's sampling frequency is 44.1kHz not 48kHz.

If you search around the net for real ABX tests, it would seem that most people can't tell the difference between 320Kb/s MP3 and 1.411 Mb/s CD.

I'd go further and say that using some lossy codecs you could go much lower than 320Kbps without audible degredation. Perhaps the difference when they occur would be more easily detected with headphones due to better resolution in general and having a 'cleaner' listening environment.

 

[eggontoast]

128, 256, 320kbps refers to compressed music bitrates, none are CD quality although there are a few people that can't hear the difference with higher bitrates.

This. maybe your confusion comes from many people saying the difference between 320kbps is and CD is inaudible.

No, there is no confusion. I would imagine that the people who can't hear any difference feel the need to post on forums about it. It probably gives them a little comfort in knowing they are not the only one who can't hear any difference. Other people who can are busy listening to their music.

 

[ID.]

128, 256, 320kbps refers to compressed music bitrates, none are CD quality although there are a few people that can't hear the difference with higher bitrates.

This. maybe your confusion comes from many people saying the difference between 320kbps is and CD is inaudible.

No, there is no confusion. I would imagine that the people who can't hear any difference feel the need to post on forums about it. It probably gives them a little comfort in knowing they are not the only one who can't hear any difference. Other people who can are busy listening to their music.

Apologies. Lazy quoting by me. I meant the OP may have been confused into thinking 320kbps is the same as CD for this reason. I should've made it clearer who I was addressing.

 

[Overdose]

128, 256, 320kbps refers to compressed music bitrates, none are CD quality although there are a few people that can't hear the difference with higher bitrates.

This. maybe your confusion comes from many people saying the difference between 320kbps is and CD is inaudible.

No, there is no confusion. I would imagine that the people who can't hear any difference feel the need to post on forums about it. It probably gives them a little comfort in knowing they are not the only one who can't hear any difference. Other people who can are busy listening to their music.

There will be a threshold at which point there is no discernable difference. That will largely depend on listening environment, and equipment. If you predominantly use headphones you are likely to have a better resolution/fidelity than speakers and your listening environment is much 'cleaner' with much less background noise, so any differences will be easier to hear.

If you listen to digital files, it makes sense to confirm what you can actually hear using a simple ABX test for peace of mind if nothing else. If you can hear differences then move on up to a level where you cannot. If you can hear no differences, keep moving down the scale until you can. Once you know your own threshold for noticing these differences, you can choose equipment and file types accordingly. Personally I don't need to go much above 192Kbps AAC using variable bit rate, although my archive is on my Mac mini in lossless format.

If you have the space, I would always recommend a lossless codec for archiving, but in all honesty, I really don't think the majority of listeners would discern 192 from CD quality. If you can then that's just fine, but iTunes and other download sites offering 256Kbps and less, don't seem to be suffering with many complaints regarding sound quality. It is a bit presumptious to think that the majority of users have cloth ears, particularly as in the main, the users are a lot younger than the typical separates hifi traditionalists and therefore have superior hearing. You could just as easily argue that the older generation are 'hearing things' pun intended.

 

[ID.]

Always lossless if I can crew checking in.

TBH I haven't blind tested it, but I've got better things to do with my time and ample storage space.

 

[Crossie]

Just an observation on CD bit rates, while CD's are capable of storing information at 1411 kb/s the vast majority do not use the full CD capability. For example my CD of "Dark Side of the Moon" in FLAC file copied losslessly replays at about 400kb/s using J River software and bypassing all the nasties in Windows. I suspect that few people could detect the difference between 400 and 320 kb/s. Of course some more modern CD recordings may have much higher bit rates few exceed 1000kb/s in my experience.

 

[andyjm]

Just an observation on CD bit rates, while CD's are capable of storing information at 1411 kb/s the vast majority do not use the full CD capability. For example my CD of "Dark Side of the Moon" in FLAC file copied losslessly replays at about 400kb/s using J River software and bypassing all the nasties in Windows. I suspect that few people could detect the difference between 400 and 320 kb/s. Of course some more modern CD recordings may have much higher bit rates few exceed 1000kb/s in my experience.

All CDs run at a constant bit rate of 1.411 Mb/s . There is no choice. There are 44.1K samples per second per channel, and 16 bits in each sample. Whether the bits are used wisely is another matter, but used or not, they are still there.

A compressed FLAC file doesnt get much smaller than about 60% of the original WAV, so the lowest bit rate you will get with a FLAC file is about 850Kb/s.

Not sure what J River is showing you.

 

[andyjm]

Always lossless if I can crew checking in.

TBH I haven't blind tested it, but I've got better things to do with my time and ample storage space.

With storage as cheap as it is, it isnt really worth compressing at all. Quite honestly, FLAC is only really worth it over WAV because of the tagging.

The only time it may matter if you are driving multiple WiFi players (Sonos for example) where WiFi bitrate is limited and compressed data can keep the bitrate down.