RE: MD: Recording MDs with microphones...
I was planning on using a topology that had the LM1973 in the *feedback* loop of the microphone preamp stage. Therefore, it would get its drive from the output of the mic preamp, after 40 dB or so of gain and attenuate it back to microphone level before feeding it to the inverting input. There's a problem with this approach. The LM1973 has a high output impedance (of the order 20k), whereas a typical microphone has a low output impedance (200Ohms or so). Thus, for best noise performance, you would ideally use a FET amplifier input for the attenuator, and a BJT input for the microphone. This presents a problem since integrated OP-amps are either one or the other. In fact, the datasheet stresses that you should use a FET at all costs, but as I said, this is a lousy ( = noisy!) match for a low-impedance microphone. You could buffer the output of the LM1973 with a FET opamp, and use a BJT opamp for the gain stage, but you'll have to watch for stability issues connecting two opamps in a loop like this. They spec these chips at over 100 dB S/N, Look closely - that's referenced to 4Vrms, which is a HUGE signal! If the self-noise is 100dB below 4V ( = +12dBV) that puts it at -88dBV. Given the standard consumer line signal level of -10dBV you've got a best-case SNR of only 78dB. Decent microphones will do a lot better than this. It is not clear from the datasheet how this SNR spec has been arrived at, so the situation could actually be a lot worse. Christopher Hicks - To stop getting this list send a message containing just the word unsubscribe to [EMAIL PROTECTED]
RE: MD: Recording MDs with microphones...
|3. Adjust levels *before* the A/D to prevent overload while maximizing dynamic | range. Most modern MD recorders have fixed gain *berfore* the A/D; not a | good thing! Absolutely. So you use the LM1973's you mentioned in a programmable gain configuration, right? Are these parts quiet enough to work with microphone-level signals? I was planning on using a topology that had the LM1973 in the *feedback* loop of the microphone preamp stage. Therefore, it would get its drive from the output of the mic preamp, after 40 dB or so of gain and attenuate it back to microphone level before feeding it to the inverting input. They spec these chips at over 100 dB S/N, and this configuration exploits the available S/N to the max, so my guess is that it would be quite acceptable. This topology is what I was refering to when I was talking about channel trim. The LM1973 contains three pots; I'd use the first one for level trim as described\ above, and the other two as conventional attenuators, one feeding the left channel and one feeding the right. That would allow the controlling microprocessor to emulate channel gain and pan, all the while maintaing an optimum gain structure to maximize S/N while minimizing overload. you're well over 1/2 watt--roughly 20 hrs. with 3-4 AA cells-- is this what you have in mind for a portable MD system? That would be great... I'd planned to use NiHM AA cells... Also, I would think that some MD titling interface built into the mixer might be a nice feature that would distinguish your mixer from anything else that's commercially available. A great idea! I'd probably use an 8051 derivative (we've had great success here at work with the Philips 87c552 in battery powered equiopment; it has some nifty power-save and sleep modes). This CPU would probably have enough resources left over to run a titling program. - To stop getting this list send a message containing just the word unsubscribe to [EMAIL PROTECTED]
Re: MD: Recording MDs with microphones...
=== = NB: Over 50% of this message is QUOTED, please = = be more selective when quoting text = === Here are some sites that have information on the LM1973 audio mixer chip. http://search.dogpile.com/texis/search?q=LM1973geo=nofs=web Timothy Stockman wrote: |3. Adjust levels *before* the A/D to prevent overload while maximizing dynamic | range. Most modern MD recorders have fixed gain *berfore* the A/D; not a | good thing! Absolutely. So you use the LM1973's you mentioned in a programmable gain configuration, right? Are these parts quiet enough to work with microphone-level signals? I was planning on using a topology that had the LM1973 in the *feedback* loop of the microphone preamp stage. Therefore, it would get its drive from the output of the mic preamp, after 40 dB or so of gain and attenuate it back to microphone level before feeding it to the inverting input. They spec these chips at over 100 dB S/N, and this configuration exploits the available S/N to the max, so my guess is that it would be quite acceptable. This topology is what I was refering to when I was talking about channel trim. The LM1973 contains three pots; I'd use the first one for level trim as described\ above, and the other two as conventional attenuators, one feeding the left channel and one feeding the right. That would allow the controlling microprocessor to emulate channel gain and pan, all the while maintaing an optimum gain structure to maximize S/N while minimizing overload. you're well over 1/2 watt--roughly 20 hrs. with 3-4 AA cells-- is this what you have in mind for a portable MD system? That would be great... I'd planned to use NiHM AA cells... Also, I would think that some MD titling interface built into the mixer might be a nice feature that would distinguish your mixer from anything else that's commercially available. A great idea! I'd probably use an 8051 derivative (we've had great success here at work with the Philips 87c552 in battery powered equiopment; it has some nifty power-save and sleep modes). This CPU would probably have enough resources left over to run a titling program. - To stop getting this list send a message containing just the word unsubscribe to [EMAIL PROTECTED] -- Jim Coon Not just another pretty mandolin picker. mailto:[EMAIL PROTECTED] If Gibson made cars, would they sound so sweet? My first web page http://www.tir.com/~liteways - To stop getting this list send a message containing just the word unsubscribe to [EMAIL PROTECTED]
Re: MD: Recording MDs with microphones...
Are referring to the CS5360 or the CS5334 ADCs? I think these chips consume ~350mW. I assume that you mix 6-8 channels of analog down to 2, then digitize. So with 6 analog channels, each with programmable gain (and possibly eq), and the other circuitry and microcontroller you're well over 1/2 watt--roughly 20 hrs. with 3-4 AA cells-- is this what you have in mind for a portable MD system? I guess that's not bad, but I would think that you would want to use Crystal's CS53L32 for this application, which brings power consumption down for this system. You lose the peak detect metering, though. Big H!! Do you guys know any average user of MD who are quite familiar with your correspondence? please, do some practical advices, opinions, or such, and spare us of your high- techie- know -how exchange of (impractical) knowledge for the most of us - To stop getting this list send a message containing just the word unsubscribe to [EMAIL PROTECTED]
Re: MD: Recording MDs with microphones...
=== = NB: Over 50% of this message is QUOTED, please = = be more selective when quoting text = === So, what would the operator of this thing use to control this mixer? Would it still have dials and slide controls like normal mixers do, or would it be hooked in a computer, or what? I thought some mixers had the ability to set presets, and motorized controls would give a tactile representation of where the controls end up when buttons are pressed. At 07:25 PM 8/1/01 -0400, you wrote: === = NB: Over 50% of this message is QUOTED, please = = be more selective when quoting text = === Here are some sites that have information on the LM1973 audio mixer chip. http://search.dogpile.com/texis/search?q=LM1973geo=nofs=web Timothy Stockman wrote: |3. Adjust levels *before* the A/D to prevent overload while maximizing dynamic | range. Most modern MD recorders have fixed gain *berfore* the A/D; not a | good thing! Absolutely. So you use the LM1973's you mentioned in a programmable gain configuration, right? Are these parts quiet enough to work with microphone-level signals? I was planning on using a topology that had the LM1973 in the *feedback* loop of the microphone preamp stage. Therefore, it would get its drive from the output of the mic preamp, after 40 dB or so of gain and attenuate it back to microphone level before feeding it to the inverting input. They spec these chips at over 100 dB S/N, and this configuration exploits the available S/N to the max, so my guess is that it would be quite acceptable. This topology is what I was refering to when I was talking about channel trim. The LM1973 contains three pots; I'd use the first one for level trim as described\ above, and the other two as conventional attenuators, one feeding the left channel and one feeding the right. That would allow the controlling microprocessor to emulate channel gain and pan, all the while maintaing an optimum gain structure to maximize S/N while minimizing overload. you're well over 1/2 watt--roughly 20 hrs. with 3-4 AA cells-- is this what you have in mind for a portable MD system? That would be great... I'd planned to use NiHM AA cells... Also, I would think that some MD titling interface built into the mixer might be a nice feature that would distinguish your mixer from anything else that's commercially available. A great idea! I'd probably use an 8051 derivative (we've had great success here at work with the Philips 87c552 in battery powered equiopment; it has some nifty power-save and sleep modes). This CPU would probably have enough resources left over to run a titling program. - To stop getting this list send a message containing just the word unsubscribe to [EMAIL PROTECTED] -- Jim Coon Not just another pretty mandolin picker. mailto:[EMAIL PROTECTED] If Gibson made cars, would they sound so sweet? My first web page http://www.tir.com/~liteways - To stop getting this list send a message containing just the word unsubscribe to [EMAIL PROTECTED] - To stop getting this list send a message containing just the word unsubscribe to [EMAIL PROTECTED]
RE: MD: Recording MDs with microphones...
On the subject of building your own mixing system for MD, what do you find lacking in commercially available mixers that would prompt you want to build your own? I can see that battery power and small size could be requirements not met by many mixers. In addition to the requirements you mention above: 1. Plug-in power for inexpensive, high quality electret microphones. 2. Reduced number of physical controls through software. (Use a few rotary encoders to replace a lot of potentiometers.) 3. Adjust levels *before* the A/D to prevent overload while maximizing dynamic range. Most modern MD recorders have fixed gain *berfore* the A/D; not a good thing! Also, why add the A/D? Are MD's converters lacking? (Or is it that the digital out is an advantage if it results in the automatic level control being bypassed.) The extremely low voltages and long battery endurance of modern portable MD players must be made at a trade-off to audio quality. Also, their small size makes proper shilelding and isolation of the analog section difficult. One other advantage of one of the Crystal Seminconductor A/D chips I've been looking at is that it incorporates a metering circuit that gives an accurate digital readout in 1 dB steps down to -70 dB. While working on a unrelated project a few years ago, I used a 16 character x 2 line LCD as a high resoltion analog bar-graph meter with software controlled ballistics, as well as for its normal text display function. What other features for MD would you look for in a mixer design? Since the proposed design would be software controlled, it could adjust the channel level trim automatically for optimum performance. The design would sort of be like a modern point-and-shoot auto-focus camera. - To stop getting this list send a message containing just the word unsubscribe to [EMAIL PROTECTED]
RE: MD: Recording MDs with microphones...
Curiously, I am currently contemplating designing a small digital mixer for location recording (not specifically to MD, but entirely usable with it). My day job is DSP design (both hardware and software) for recording studio equipment (www.cedaraudio.com); my hobbies are performing and recording classical music (oh, and furniture making...). It recently occurred to me that a small digital mixer is quite feasible as a DIY project with careful design. Other complicated bits (such as FPGA designs and firmware) can be distributed in binary form. 3. Adjust levels *before* the A/D to prevent overload while maximizing dynamic range. Most modern MD recorders have fixed gain *berfore* the A/D; not a good thing! It is technically correct that to maximise ADC performance you should do the gain control before the ADC; however, with modern ADC chips you can get extremely good performance (up to about 100dB dynamic range, depending on details) with this analogue gain control being pretty crude (I am planning 3 or 4 settings spanning approximately a 50dB range). I agree that most small MD recorders do this to keep the cost down with little regard to audio quality, and probably don't even have the crude analogue gain switching that I suggest. The performance I'm aiming for will not be the same as that obtained with truly professional ADCs costing megabucks, but the DSP will be second to none and I expect the unit overall to be good enough for producing commercial CD releases. The unit will be portable but not battery powered. I'd expect the parts cost to be of the order of $400 to $500 for a 1-off. My project essentially comprises 8 XLR mic-line inputs, each with switchable gain and phantom power. The resulting audio is passed to a DSP which has independent level and pan controls for each channel, a master level control, and a parametric EQ on the output. It would be possible to have EQ on individual channels but it would make the control surface either expensive, or fiddly to use, and I'd rarely use it in my application anyway. As well as the digital master out (dithered/truncated as necessary) there is a separate analogue monitor out which will drive headphones, but which is really designed to drive a pair of active monitor speakers directly. This monitor out normally follows the main mix, but also allows solo and PFL functions which don't interfere with the main mix (which is what gets recorded). There will be decent level meters on the monitor out. The control surface will probably comprise 9 faders (8 inputs plus a master), plus an LCD display, soft keys and opto encoder to set other parameters. The make-or-break of this project probably depends on whether I can get a hobbyist electronics magazine to publish the design. The purpose of this posting is too see if there is significant interest in this forum that I can use as leverage with potential publishers, so let me have your comments! While working on a unrelated project a few years ago, I used a 16 character x 2 line LCD as a high resoltion analog bar-graph meter with software controlled ballistics, as well as for its normal text display function. Good idea, and one I have used too! Christopher Hicks - To stop getting this list send a message containing just the word unsubscribe to [EMAIL PROTECTED]
Re: MD: Recording MDs with microphones...
Christopher Hicks wrote: Curiously, I am currently contemplating designing a small digital mixer for location recording (not specifically to MD, but entirely usable with ... My project essentially comprises 8 XLR mic-line inputs, each with switchable gain and phantom power. The resulting audio is passed to a DSP which has independent level and pan controls for each channel, a master level control, and a parametric EQ on the output. It would be possible to have EQ on individual channels but it would make the control surface either expensive, or fiddly to use, and I'd rarely use it in my application When I record a group, I usually put up 4 mics in an A-B X-Y configuration, and then the occasional spot mic when needed. I often use eq on the individual channels to blend the sound from the mics, so I'd really love it if there were individual channel eq. The make-or-break of this project probably depends on whether I can get a hobbyist electronics magazine to publish the design. The purpose of this posting is too see if there is significant interest in this forum that I can use as leverage with potential publishers, so let me have your comments! I would absolutely love something like this. Even without the eq on individual channels. The other thing I'd love (while you're in the designing mood...) is a small portable D/A converter with an eq in it. It would take in optical or coax S/PDIF and then have a digital eq, and then output a line level signal. Perhaps if you just added a digital input to your mixer above, which would also be useful as an alternate clock source. -steve - To stop getting this list send a message containing just the word unsubscribe to [EMAIL PROTECTED]
RE: MD: Recording MDs with microphones...
|-Original Message- |From: Timothy Stockman [mailto:[EMAIL PROTECTED]] |Sent: Monday, July 30, 2001 6:40 AM |Subject: RE: MD: Recording MDs with microphones... | snip | |1. Plug-in power for inexpensive, high quality electret microphones. |2. Reduced number of physical controls through software. (Use a few rotary | encoders to replace a lot of potentiometers.) So you need an LCD display, too, as part of the user interface. |3. Adjust levels *before* the A/D to prevent overload while maximizing dynamic | range. Most modern MD recorders have fixed gain *berfore* the A/D; not a | good thing! Absolutely. So you use the LM1973's you mentioned in a programmable gain configuration, right? Are these parts quiet enough to work with microphone-level signals? | |One other advantage of one of the Crystal Seminconductor A/D chips I've |been looking at is that it incorporates a metering circuit that |gives an accurate |digital readout in 1 dB steps down to -70 dB. While working on a unrelated |project a few years ago, I used a 16 character x 2 line LCD as a |high resoltion |analog bar-graph meter with software controlled ballistics, as |well as for its normal |text display function. Are referring to the CS5360 or the CS5334 ADCs? I think these chips consume ~350mW. I assume that you mix 6-8 channels of analog down to 2, then digitize. So with 6 analog channels, each with programmable gain (and possibly eq), and the other circuitry and microcontroller you're well over 1/2 watt--roughly 20 hrs. with 3-4 AA cells-- is this what you have in mind for a portable MD system? I guess that's not bad, but I would think that you would want to use Crystal's CS53L32 for this application, which brings power consumption down for this system. You lose the peak detect metering, though. | |Since the proposed design would be software controlled, it could adjust the |channel level trim automatically for optimum performance. The design would |sort of be like a modern point-and-shoot auto-focus camera. | Sorry, I'm not sure what you mean here? Also, I would think that some MD titling interface built into the mixer might be a nice feature that would distinguish your mixer from anything else that's commercially available. - To stop getting this list send a message containing just the word unsubscribe to [EMAIL PROTECTED]
RE: MD: Recording MDs with microphones...
On the subject of building your own mixing system for MD, what do you find lacking in commercially available mixers that would prompt you want to build your own? I can see that battery power and small size could be requirements not met by many mixers. Also, why add the A/D? Are MD's converters lacking? (Or is it that the digital out is an advantage if it results in the automatic level control being bypassed.) What other features for MD would you look for in a mixer design? |-Original Message- |From: Timothy Stockman [mailto:[EMAIL PROTECTED]] |Sent: Sunday, July 22, 2001 1:22 PM |Subject: MD: Recording MDs with microphones... | | SNIP | |I've had the ambitious thought of building up my own mixing |system, consisting of OPA2604 |amplifiers and LM1973s for electronic level control and panning, |using some sort of CPU to |control it all and maybe even an A/D chip so I can feed digital to the MD. | |Has anyone else on the list though about something like this? | - To stop getting this list send a message containing just the word unsubscribe to [EMAIL PROTECTED]
