Q: Op-amp for high quality voice ??

[mas1738]

Hi Guys I want to design an optical transmitter where i want my laser to
send the data in digital form ; however, the 741 chip ( as a comparator on the reciever) can not handle high rate (20KHz), so i want to know if there is an op amp that can work at 6bits transmission rate per sample where the voice range up to 20kHz ???

 

[duffy]

The LM6132 can handle it, and the output gives you more dynamic range.

 

[crutschow]

It's better to use a dedicated comparator IC such as an LM339, which has a 0.5µs resonse time, rather than an op amp which has compensation circuitry that slows it down.

If that speed isn't adequate, National Semi sells other faster units.

 

[audioguru]

I am glad that you know that high quality voice has frequencies up to 20kHz, unlike a "what did you say?" telephone or AM radio that cuts off important frequencies above only 3kHz or less.

 

[mas1738]

Thank you guys very much , but my lecturer said LM358 will do the job , i am not
sure about ,so i will try it in the lab .

 

[audioguru]

The lousy old LM358 is a low power dual opamp so it has a poor high frequency response at full output up to only 2kHz, and its output transistors do not have enough bias current so they operate in class-B instead of in class-AB so it has 3% of crossover distortion. It is also noisy.

I use TL072 dual low noise, low distortion and wide bandwidth opamps that cost about the same or less.

 

[Nigel Goodwin]

Thank you guys very much , but my lecturer said LM358 will do the job , i am not
sure about ,so i will try it in the lab .

It's all that's required for 'voice', which doesn't need to be good quality anyway - assuming we're talking about 'talking' and not singing?.

What is your assignment?.

 

[audioguru]

A voice should sound clear, not muffled and distorted like a guitar.

 

[Nigel Goodwin]

A voice should sound clear, not muffled and distorted like a guitar.

Assuming it's just for communication there's no need for extended response, even here though distortion is best avoided - but even assuming you get 3% from an LM358, it's plenty low enough for speech (think of it as a valve amplifier ).

 

[crutschow]

To paraphrase, mas1738 initially stated he was trying to use a 741 as a comparator in the receiver to recover the detected digital optical pulses, but the 741 was too slow. Seems like the thread has been sidetracked into a discussion of the analog speech performance of an op amp, which is unrelated to the comparator problem.

Or am I missing something?

 

[audioguru]

"High quality voice" and "voice range up to 20kHz" were mentioned by the OP. He also said that the horrible lousy old 741 opamp was not good enough.

Then he said that the lecturer said that an LM358 (which is worse than a 741) was good enough.
Then throw "high quality" and "20kHz" out the window and make a low quality muffled distorted "what did you say?" telephone.

I listen to FM radio a lot. The music is old and new and is very clear. The announcers' voices are also very clear. AM radio has such poor sound quality that I hardly ever listen to it.

 

[Nigel Goodwin]

"High quality voice" and "voice range up to 20kHz" were mentioned by the OP. He also said that the horrible lousy old 741 opamp was not good enough.

He also said he wanted 6 bits resolution

Then he said that the lecturer said that an LM358 (which is worse than a 741) was good enough.
Then throw "high quality" and "20kHz" out the window and make a low quality muffled distorted "what did you say?" telephone.

An LM358 is still perfectly good enough for decent quality voice use, six bits resolution is far more limiting.

I listen to FM radio a lot. The music is old and new and is very clear. The announcers' voices are also very clear. AM radio has such poor sound quality that I hardly ever listen to it.

I never do, can't remember the last time I did?.

 

[audioguru]

Roman Black has a one bit encoder/decoder that sounds pretty good.
Many years ago I heard an 8-bits telephone call that sounded horrible.

 

[Nigel Goodwin]

Roman Black has a one bit encoder/decoder that sounds pretty good.
Many years ago I heard an 8-bits telephone call that sounded horrible.

Except that isn't what '1 bit' means - obviously true 1 bit can only carry simple binary information, either ON or OFF.

 

[Hero999]

my lecturer said LM358 will do the job

He obviously doesn't know what he's talking about. The LM358 is no better than the 741. I think you should seek tuition from someone who knows what they're talking about.

 

[mas1738]

I think you should seek tuition from someone who knows what they're talking about.

Ha-ha you are right hero999 I test the op amp and the performance was bad, and actually he don’t know anything I don’t know how he got his PHD.

What is your assignment?.

Its an optical transceiver which transmit sound signal up to 20kHz, so it should be 20kHz

 

[audioguru]

Some deaf people say that voices have frequencies up to only 3kHz. (What did you say?)
Experts measured voices going up to 14kHz and higher.

 

[Nigel Goodwin]

Ha-ha you are right hero999 I test the op amp and the performance was bad, and actually he don’t know anything I don’t know how he got his PHD.

If you want good audio quality you need a better class of opamp, and plenty are available, even cheaper than the antiques you're using - the TL0xx series are great value.

Its an optical transceiver which transmit sound signal up to 20kHz, so it should be 20kHz

So what has 'six bits' got to do with it?.

If you're digitising the audio with only six bits, then quality is going to be absolutely awful, regardless of frequency response.

 

[Hero999]

Ha-ha you are right hero999 I test the op amp and the performance was bad, and actually he don’t know anything I don’t know how he got his PHD.

He might know a lot about electrical theory but obviously knows nothing about real components and how to interpret a datasheet.

 

[mas1738]

So what has 'six bits' got to do with it?.

If you're digitising the audio with only six bits, then quality is going to be absolutely awful, regardless of frequency response.

So you advice me not to digitize the signal, because the project will be marked by measuring the SNR at the receiver so if the analog signal will give me better SNR I will go for it, since I didn’t build the circuit yet

Some deaf people say that voices have frequencies up to only 3kHz. (What did you say?)

As far as i know that the human voice has a frequency up to 3.4kHz however the human can hear up to 18-20kHz (for example: music)