Audio Conventions - Levels and Meters - Page 1
Forum section: electronics / beginners

author: ed_reardon
author_rank: Regular Contributor
timestamp: May 27, 2018, 09:13:23 am
content: Hello all,Starting out on a little project, wonder if anybody can help here, as I'm getting nowhere fast talking to experts, I must be being obtuse! I'm looking at building a small mixing desk and audio recorder.The idea is to have selectable inputs between microphones with conventional TRS connectors as well as powered XLR inputs. Can anybody clarify which are the conventional nominal levels used between the two connectors and types.Also for metering on a mixing console, do the conventional slider controls with the 'chasing' LEDs measure a rectified average, peak or VU because again, I can't really get a clear answer. I've had many 'dunnos' but to me there's quite separate concepts!Sorry if these are really daft questions, but audio is something I've never touched. It's only for a little project, I know I could buy something cheap from eBay, but where's the experience and challenge in that!?Cheers,Ed

author: Harb
author_rank: Regular Contributor
timestamp: May 27, 2018, 09:29:01 am
content: You will do very well to nail down a standard in Audio........ There are so many "Standards" depending on who you talk to.....I am in the TV industry, so the only one we are interested in is +4dBm (600 Ohms) after all is said and done....it used to be +8 a few years ago, but +4 is now the accepted level....Having said that, with the acceptance of HD things have become different again and measuring much more complex.

author: ed_reardon
author_rank: Regular Contributor
timestamp: May 27, 2018, 09:33:03 am
content: Quote from: Harb on May 27, 2018, 09:29:01 amThank you Harb, yes, I'm starting to realise that nobody seems entirely sure what's-what!

author: Harb
author_rank: Regular Contributor
timestamp: May 27, 2018, 09:35:02 am
content: Audio is awful.......Domestic gear is all over the place.......

author: dmills
author_rank: Super Contributor
timestamp: May 27, 2018, 12:16:01 pm
content: Mic levels can be all over the place (apart from anything else depending on where you PUT the things!), which is why there is usually a mic gain control with at least a 40dB range (and often a 20dB pad switch as well), maximum gain wants to be somewhere around 60dB for a general purpose desk.Channel strip meters should generally be peak responding as your major use here to to make sure you are not clipping (These meters should be pre-fader), a clip indicator sampling several points in the strip is also a good idea.Output meters very much depend on what you are intending to drive, PPM is traditional on EU broadcast desks, but if you are driving some sort of digital chain a very fast attack DPM style affair may be more useful, or if working in a loundness normalised environment a EBU style BS1770 job may be appropriate.Nominal levels at line outputs are usually a few volts, but often have at least 20dB of headroom over whatever nominal means.Were I doing this project I would start by obtaining a copy of Doug Selfs "Small signal audio design" book, well worth the investment. Focal press also have an entire book on audio metering that may be relevant.Regards, Dan.

author: Benta
author_rank: Super Contributor
timestamp: May 27, 2018, 01:16:14 pm
content: The only "standard" I'm aware of is 0 dBm (= 1 mW) into 600 ohms at line level, which works out at around 775 mVRMSfor 0 dB. You'll need headroom of course, at least +6 dBm, +12 is better.

author: BrianHG
author_rank: Super Contributor
timestamp: May 27, 2018, 01:48:13 pm
content: For the line levels, tadaa:https://en.wikipedia.org/wiki/Line_levelIt is ugly with 4 reference levels as others have already reported on this thread.When Vu metering, use all these possible references depending on your application and Vu like a true-rms AC volt-meter, reporting the db difference from the true level.As for how your meter peaks, the response of your meter should be a true-rms voltage based, but, having a peak spot should be filtered to you visual liking and how you want the response impact.Going all digital is easier as the either 32bit-float, 32bit-int, 24bit or 16bit levels VS intended signal db are well defined.Also, using an MCU/dspic/DSP to either read the digital or adc at 44.1khz creating a quality VU meter display is a valid approach so you can make true-rms and power calculations and choose you meter's scale and grade at software will. A 3$ dspic has a deep enough 12bit adc at 1mhz sampling/channel, enough for 2 or 4 channel super programmable quality VU meters.

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 27, 2018, 03:25:04 pm
content: Quote from: ed_reardon on May 27, 2018, 09:13:23 amIt is rather rare here in the 21st century to find microphones with 1/4 inch phone connectors, either TS or TRS. Virtually all microphones in modern times are using either "professional" XLR connections, or "amateur" 3.5mm (as for portable gear like consumer camcorders, voice recorders, etc.)If you could clarify what you are doing with this and what kind of microphones you want to accommodate, it would be easier to answer your question.Quote"Pro" XLR mic inputs are often provided with the (switchable) option of Phantom Power. This started out as 48V (for historic reasons), but most modern microphones which require phantom power will operate on lower voltages like 24V or even 12V.QuoteXLR connectors are commonly used for both line-level and mic-level signals. In many cases the level of the XLR input (or output) is user-selectable for mic-level or line-level. In very rough, back-of-the envelope terms, you can think of line-level as 1V, and mic-level as 1mV. That is why typical mic preamp circuits have around 60dB of gain. (1000x)1/4 inch connectors are used primarily for four different kinds/levels of audio signals:Line-level.Same signals as used with XLR. Generally XLR and TRS line-level connections are equivalent and interchangeable. Of course TRS allow for a balanced signal interface while TS connectors support only unbalanced.Instrument-level.This is the signal that comes out of an electric guitar (or bass, etc.) Traditionally quite high-impedance (~100K vs. 1K-10K for line-level) and quite low signal-level (equivalent to mic-level)Headphones.In modern times TRS, but stereo, unbalanced with common/ground on Sleeve, and Left channel on Tip, and Right channel on Ring. Signal levels roughly equivalent to line-level and quite low impedances 20-150 ohmsEffects Insert.Used in mixers as an "interrupt" to the normal path. So that the channel signal can be pulled out an sent through an external process (limiting, compression, reverb, etc.) and then back into the channel. A "closed-circuit" TRS jack is used where the Send (Ring) is connected across to the Return (Tip) wen nothing is plugged in. Signal levels are typically at (or slightly lower than) line-level.QuoteWhile there are some (rare) examples where LED arrays show the "setting" of a slider control, they are typically used to meter the audio levels regardless of whether slider controls are used or not. I will assume you are asking about audio level monitoring and not control position(?)QuotePerhaps because there are many ways of doing that depending on the application, the budget (cost of the gear), etc. For critical, professional (broadcasting, studio recording/mixing) applications LED meters frequently have special "ballistics" and calibration. And even at that, there are different standards in America vs. European practice. So it somewhat depends of where you are as well.If you are just doing a small hobby project, simply start with something simple like a fast-responding "peak" reading meter circuit. When you start playing with it you will see the advantages and disadvantages of peak-reading vs. averaging (RMS, traditional "VU") metering of audio signals. Don't sweat it. Start with something simple and improve/enhance your circuits as you gain experience with design, construction and (most importantly) actually using the gear.A very popular (and easy) solution for LED-string audio metering are the TI chips LM3915 and LM3916. I very strongly suggest downloading the data sheets for these chips. They enable you to construct a quite decent meter with the single chip, a string of LEDs, and a few resistors and capacitors.Also, the LM3915/16 data sheet and application notes show audio signal processing circuits that implement: Half-Wave Peak detector (Fig. 17), Precision Half-wave Rectifier (Fig. 18), Precision Full-Wave Average Detector (Fig. 19) and Precision-wave Peak Detector (Fig.20)Many pieces of commercial audio gear use these chips (or equivalent) to implement LED-ladder style audio-level metering.Note that there are hundreds of reference designs out there on the internet for mic-level and line-level input and output circuits. Of particular interest are the products made by THAT Semiconductor and many specialized audio chips made by TI and Analog Devices. There are also schematic diagrams for many popular audio mixers available for study (or home construction).LM3915/16:https://media.digikey.com/pdf/Data%20Sheets/Texas%20Instruments%20PDFs/LM3814,15.pdfTHAT Semiconductor:http://www.thatcorp.com/THAT_IC_Products.shtmlTI Audio ICs:http://www.ti.com/audio-ic/overview.html?keyMatch=audio&tisearch=Search-EN-EverythingAnalog Devices ICs:http://www.analog.com/en/applications/markets/consumer-pavilion-home/proav.html

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 27, 2018, 03:32:25 pm
content: Other resources for audio circuit examples include:Elliot Sound Products:http://sound.whsites.net/index2.htmlGyraf Audio:http://gyraf.dk/diy-projects/DIY Audio:http://www.diyaudio.com/

author: dave_k
author_rank: Frequent Contributor
timestamp: May 27, 2018, 11:33:14 pm
content: Quote from: Harb on May 27, 2018, 09:29:01 amIn my 20 or so years in broadcast, both TV and radio, I haven't seen +8dBm used as a reference level (except for the old Fox FM studios in Melbourne). Going further I haven't even seen +4dBm used as a reference very much.Most professional audio equipment nowadays feature low source impedance and high input impedance, so +4dBu is almost universally used.

author: Harb
author_rank: Regular Contributor
timestamp: May 28, 2018, 01:48:20 am
content: Quote from: dave_k on May 27, 2018, 11:33:14 pmI've been in a while too Dave....to long lol 26 years this year, I still like it, but miss the days of long haul microwave links and getting to sit on a hill somewhere and relaxing......again this just shows how varied the so called standards are.....I have level meters here that were built by the broadcaster/s that were calibrated for +8dBm and later changed to +4dBm as they started to realise Valve gear was changing over to solid state and overcoming noise figures was less of a drama......we still are requested to feed use a +4dBm 1K tone into the remaining analogue feeds .

author: ed_reardon
author_rank: Regular Contributor
timestamp: May 28, 2018, 08:29:15 am
content: My gratitude to all of you, there's some great information here for me to go and research!Thanks again,Ed

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: May 28, 2018, 08:58:44 am
content: Standard line level is 0.775 volt RMS, which is also 1v peak, and 600R source impedance. Sounds like that ought to be simple, but the problems mostly arise from the European DIN standard which specifies a high source impedance such that the actual voltage depends a lot on the load.Metering is either VU or PPM. VU meters respond to average voltage (a simple half wave unfiltered rectifier) and 0dB is the correct recording level for the loudest passage. PPMs respond to peak voltage (both in the sense of Vp for the waveform and the max encountered over a time interval of a few hundred ms) and are scaled 0-5. The correct recording level is typically 3.

author: dave_k
author_rank: Frequent Contributor
timestamp: May 28, 2018, 09:00:37 am
content: This is a good reference chart when it comes to matching audio levels across different areas of the globe.

author: ed_reardon
author_rank: Regular Contributor
timestamp: May 28, 2018, 10:36:15 am
content: Cheers again all.As I want to use this as a mixer-recorder (it's essentially going to be a small 4-channel mixing desk with a built in MP3 codec and SD card) unless I've totally misunderstood I can set 0dB at the typical V(pp) of the line-input in to the encoder IC:Typical Line Input = 2500mVpp= 1250mV half-wave peak.Using an LM3915 with the half-wave peak detector and 1250mV full scale value should yield a usable meter, there's a bit of head-room with the numbers as harmonic distortion doesn't become apparent until 2800mVpp by the spec' sheet.By my reckoning this should be okay for setting channel gain, this doesn't need to be broadcast standard driven, so as Dave says, you can always make up your own dB scales!Hope this makes sense, audio really isn't something I've thought much about.

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 28, 2018, 11:27:25 am
content: You did not mention what you are connecting this to? If it is typical consumer or prosumer gear, then 2.5V is very hot for nominal working levels. Consumer standard is -10dBu (around 0.5VPK)Note that the data sheet specs for your (unidentified) MP3 chip is probably the max limit, and not the nominal standard operating level.Ref:https://en.wikipedia.org/wiki/Line_level#Nominal_levels

author: ed_reardon
author_rank: Regular Contributor
timestamp: May 30, 2018, 09:03:20 pm
content: Sorry, I maybe should have mentioned the IC!It's the VLSI VS1053, datasheet herehttp://www.vlsi.fi/fileadmin/datasheets/vs1053.pdfPage 8 gives the values I'm looking at.Cheers,Ed.