Welcome to No Limit Sound Productions

Company Founded

Our services include Sound Engineering, Audio Post-Production, System Upgrades and Equipment Consulting.
Our mission is to provide excellent quality and service to our customers. We do customized service.

Saturday, February 25, 2017

Q. Which PC laptop should I buy?

By Martin Walker
Dell's notebooks offer good specs at decent prices, but you may find that they are not ideal for audio. 
Dell's notebooks offer good specs at decent prices, but you may find that they are not ideal for audio.

I'm considering buying a laptop for various audio tasks, including recording school choirs, community music workshops, live DJ'ing (using Ableton Live), and writing my own material. I'm looking at a Dell Inspiron 9400 and think I've made my mind up but I was just wondering if you had any advice on this particular machine and if I should be considering any others?
Lee Perry

PC Music Specialist Martin Walker replies: Thanks for the compliment, Lee — we do our best! As one of the first models featuring Intel's Centrino Core Duo processor range, Dell's Inspiron 9400 will certainly be fast enough to run lots of plug-ins and soft synths and perform plenty of audio manipulations in Ableton Live, whether you choose the 1.86GHz or 2.0GHz model. For recording school choirs and community music workshops I suspect you'll want a quiet laptop, so that no cooling fan noise is picked up by the microphones, and once again a Centrino model should be ideal.

The Inspiron 9400's 17-inch widescreen display and high-end graphics with 256MB of dedicated RAM is generous, to say the least. However, choosing a model with such a large screen does have one disadvantage — weight. At 3.6kg (4.2kg with the AC adaptor) the Inspiron 9400 is generally regarded as too heavy for regular travel which, judging by your list of proposed activities, may influence your final decision.

Don't forget also that you've left out the most important gear item in your query — the audio interface. Recording school choirs and community music workshops could require plenty of microphones, so in a way you're putting the cart before the horse — perhaps choosing a suitable multi-channel interface should come first. Indeed, were you only to be carrying out these activities, many people wouldn't recommend a laptop at all, instead favouring some sort of hard disk recorder or digital multitracker with built-in mixer and mic preamps, so you'd end up with less boxes to cart around.

However, with your DJ'ing and own music composition in mind, let's return to the proposed laptop. Many musicians would look down their noses at one aspect of its performance — the hard drive. At 5400rpm, this seems out of sync with the rest of the specification. Although I'm perfectly happy with the performance of the Seagate Momentus 5400rpm hard drive in my own laptop, which is capable of recording and playing back at least 30 audio tracks at 24-bit/96kHz, most musicians nowadays tend to look for laptops with 7200rpm hard drives, particularly if they intend to record using the more demanding 24-bit/96kHz audio format.

Of course, like lots of other musicians, you could add an external 7200rpm hard drive, but that's yet another box to transport to your live recording sessions. This brings us to another hurdle for PC-music laptop performance — the Firewire controller chip found on the laptop's motherboard that's in charge of Firewire duties, which can be crucial to achieving glitch-free Firewire audio recording and playback.
There are three main audio interface formats suitable for laptop use: USB, Firewire and PCMCIA. Since the Dell Inspiron 9400 features one of the new Express Card 54 slots rather than a PCMCIA one, you're restricted to the first two, but fortunately buying a Centrino-based laptop guarantees you an Intel USB controller for maximum compatibility with USB audio interfaces. However, there are still very few multi-channel USB 2.0 audio interfaces available, so the majority of musicians who need multi-channel recording capability opt for a Firewire audio interface.
Specialist laptops like this one from Nu Systems are custom-made with audio in mind. 
Specialist laptops like this one from Nu Systems are custom-made with audio in mind.

It's generally accepted that Texas Instruments' controller chips are more widely compatible with Firewire audio devices, and most interface manufacturers provide some sort of list of compatible chips on their web sites. However, it can be difficult to find out which Firewire controller chip a particular laptop uses, unless its manufacturer is prepared to tell you — and a mistake here will be expensive to rectify.

One final factor can prove very frustrating for musicians when choosing a laptop: ground loops. Many manufacturers use three-wire earthed power supplies for their laptops, particularly if they feature metal cases, and this extra earth connection is notorious for causing ground-loop problems such as background buzzes, whistles and other noises in your audio, which change during hard drive activity, when you move your mouse, and during graphic redraws. I'm afraid Dell are one of the most notorious in this respect, and I specifically mention the Inspiron 8200, 8500 and 8600 models as causing such problems in my PC Music FAQs forum, at www.soundonsound.com. You can cure such problems with a systematic approach and DI boxes between your audio interface and other gear, but it's added frustration and expense.

For this reason, I generally recommend one of two approaches when choosing a PC laptop for music. The safest is to contact a specialist music retailer (many of whom advertise in the pages of SOS) and discuss your requirements with them, as they not only choose their laptop components carefully for maximum compatibility and minimum likelihood of ground-loop problems, but can also advise on the most suitable audio interface to go with the laptop, depending on what tasks you want to perform.
You may pay a little more than you would for a mass-market model, but you'll get technical backup from experts who know all about music hardware and software — after all, most mainstream laptop manufacturers have little or no idea of the particular technical requirements of the musician, which are rather different from most ordinary users. I bought my Centrino laptop from a specialist music retailer, as have members of SOS staff, simply to avoid all the potential pitfalls and possible strife.

The second approach is to post your suggested laptop model on the SOS Forums and ask if any other musician has bought it, and, if so, what experience they have had. Such answers are, of course, no guarantee of a problem-free purchase, and you're likely to have the added confusion of a host of other models being recommended as alternatives, and others to avoid, but at least this approach is better than buying blind. Whatever model you decide on, choose your audio interface at the same time, and try to buy from a dealer who is prepared to give you a refund or exchange on the laptop in the (hopefully unlikely) event of the two ending up in conflict.

Ultimately there are hundreds (possibly thousands) of different PC laptop models available, all including a slightly different combination of components such as motherboard, CPU, RAM, hard drive, graphics, screen, selection of ports, and so on. Such competition keeps prices keen, but it does make choices more difficult for the musician!

Published July 2006

Wednesday, February 22, 2017

Q. Why is my Dual Tube Channel so noisy? Is it a faulty unit?

By Hugh Robjohns
Mindprint's Dual Tube Channel (DTC) remains the top choice of many professional users. 
Mindprint's Dual Tube Channel (DTC) remains the top choice of many professional users.

I recently purchased a Mindprint DTC and I've noticed that it produces quite a lot of hiss. It is not so obvious to the ear at first but after individually compressing instruments and playing them together, it becomes quite nasty. Also, if you are monitoring the signal from the DTC, you notice quite a large amount of noise on the analyser. I purchased the 24-bit S/PDIF module to find the same happens there. Whilst I was sold this as a mastering tool I am wondering if this is a design flaw. I have read some good reviews (including yours from SOS June 2002), but on a few web sites, similar problems are mentioned. I have done all the regular troubleshooting, such as changing cables and checking the power distribution of my system, and it all looks fine. I am wondering why my DTC is so noisy and if it is supposed to be.

Chris Frost

Technical Editor Hugh Robjohns replies: This is tricky to answer without actually hearing the problem you are complaining about and knowing how you are using the product. However, the DTC remains a favourite processor of mine, and I've not had any serious noise problems when I've used it, so I would suspect either an operational problem or a faulty unit.

As it uses valves, it is possible that you have a faulty one, which could lead to excessive noise. Changing the valves is not difficult and replacements aren't expensive — but it might be worth getting the product properly checked-over by a qualified technician, in case anything more serious is wrong. Unlikely, but it's always best to get it checked.

Perhaps the more likely problem is an operational one. Setting an appropriate gain structure is important to optimise the signal levels through the unit. The other thing that intrigues me is that you say: "It is not so obvious to the ear at first but after individually compressing instruments and playing them together, it becomes quite nasty". Noise will always add and build in level, so processing individual instruments with the DTC will always produce a noisier result than simply processing the final mix. But I wonder if, in fact, you are over-compressing the instruments.

Ideally, you could send a short extract of some affected material, but it may be easier and quicker to get the unit checked-out or compared with another unit to make sure all is working as it should.
I've always found Mindprint to be helpful in resolving issues like this, so giving them a call might be a worthwhile step to take as well. Mindprint +49 6851 9050.

Published October 2006

Monday, February 20, 2017

Q. When I mix in Cubase, my mixed-down songs do not sound as good in playback as they did before I saved them. Why?

By Hugh Robjohns
Q. When I mix in Cubase, my mixed-down songs do not sound as good in playback as they did before I saved them. Why?

When I mix in Cubase, my mixed-down songs do not sound as good in playback as they did before I saved them. I listen before and after saving; in the same room with the same monitors; with the same quality conversion, and without any digital clipping on any busses. I usually mix 32 channels down to eight stems, and then into a stereo output buss.

Does analogue summing provide a more accurate representation of the multi-channel mix, an enhancement to the sound of the mix, or both? Can I get to the first option by simply capturing the pre-save monitor output with an accurate stereo A-D converter? And can most of the benefit of analogue summing be derived from creating stems in the DAW before saving, then routing just the stems out for analogue summing? Alternatively, can those benefits be gained by making all adjustments to channel levels in the software, and then running the software-adjusted signals through a summing device that makes no further gain adjustment? If so, are the gain controls on most analogue summing devices necessary?

Vince Daukus

Technical Editor Hugh Robjohns replies: Wow, that's a lot of questions to get through! Bear in mind that I'm not a Cubase expert, so I can only talk to you from the analogue summing perspective.
The first query, about an audible difference between a 'live' session and a replay from the saved file, is an interesting one. The audio components of a session in progress will typically be working within a 32-bit floating-point domain, whereas the output D-A will only accept a 16- or 24-bit fixed-point signal. So a conversion process is required which will involve some form of dithering. It is well known that some dithering algorithms have a more audible effect on some material than others — particularly where some kind of noise-shaping is involved. So it is possible that the differences you hear are the result of the conversion from the floating-point format to a truncated and dithered fixed-point one.

However, it is probably more likely that the differences you hear are due to the way you are monitoring the 24-bit mixed file. If you are using a different program to replay these files, that alone may explain the change — perhaps there is some kind of internal processing going on that has an audible effect. If you are monitoring through Cubase, it might be because the file is being transcoded back into a 32-bit floating point format within Cubase, and then converted yet again to 24-bit for the monitoring output. That kind of multiple conversion process is likely to result in a sonic degradation, no matter how subtle.

Moving on to analogue summing, I would say it generally sounds 'different' rather than 'better.' Analogue summing is inherently less technically accurate than digital summing (when performed correctly), but often those technical imperfections are musically and sonically attractive — in much the same way that the high levels of harmonic distortion created by some valve preamps are pleasing to the ear. So the answer to your question is: yes, in some circumstances analogue summing enhances the sound of a mix.
Rackmount summing mixers are a relatively new phenomenon. These two models have been reviewed in SOS recently; the Audient Sumo (above) in February 2006 and the AMS Neve 8816 in August 2006. 
Rackmount summing mixers are a relatively new phenomenon. These two models have been reviewed in SOS recently; the Audient Sumo (above) in February 2006 and the AMS Neve 8816 in August 2006.

One thing that analogue summing provides for free is mix headroom. A huge number of people 'mixing in the box' do so with very high peak levels on each source channel; typically well up above -6dBFS. This results in the mix-buss accumulator having to accommodate signals with sample values above 0dBFS and the master output fader has to be pulled down to compensate. Although this absence of mix headroom shouldn't be a problem in theory, it appears that some systems (and plug-ins) do have a problem with it, and a great many users have found that leaving more headroom on the source channels avoids the problem completely, producing far sweeter-sounding mixes as a result. The old analogue concept of having 20dB or so of headroom was arrived at for a very good reason and has stood the test of time. The concept translates perfectly to the digital world, with the same practical and sonic advantages.

You asked if you could capture the pre-save monitor output with an accurate stereo ADC. Well, yes, you could, but it won't improve anything on a technical level. The output from Cubase is transcoded internally from 32-bit floating point to 24-bit fixed-point samples. These are then passed to the digital output and in turn to the D-A converter that feeds your monitoring. Routing that analogue signal back through yet another A-D converter to get it back into the digital domain gains nothing at a technical level and is likely to degrade rather than enhance the signal. Of course, it is possible that you may like the sonic character imposed by that degradation, in the same way that people like valve preamps.

Next you asked if most of the benefit of analogue summing can be derived from first creating stems in the DAW before saving. The answer is a qualified yes. The advantages of mix headroom and the sonic colouration will be the same whether you mix individual channels or stems in an outboard analogue summing box. However, if the mixing of stems created within Cubase produced samples greater than 0dBFS, the quality loss resulting from a lack of mix headroom in the digital domain would remain.

Finally, you asked if most of the benefit of analogue summing could be derived by making all adjustments to channel levels in the software, and then running the software-adjusted signals through a summing device that makes no further gain adjustment? The answer here would be yes again, because by adjusting the level of source channels before mixing, the advantages of mix-headroom are preserved, and the mix may well sound better as a result.

The gain controls on those analogue summing devices that include them are there to allow the user to optimise the gain structure for whatever source equipment is being used. Some D-A converters operate with peak levels of +26dBu, while some only reach +12dBu, so an input gain trim to optimise the gain structure is both necessary and important if you can't determine the source input levels.

Published November 2006

Friday, February 17, 2017

Q. What is a Ground (Earth) Loop?

By Hugh Robjohns
Lifting the mains earth like this could lead to serious injury.  
Lifting the mains earth like this could lead to serious injury.

Years ago I touched a guitar that was plugged into an amp against a radiator, and it blew the amp and melted the guitar string. Afterwards someone told me that this was caused by a ground loop, but I've never actually understood what that means.

SOS Forum Post

Technical Editor Hugh Robjohns replies: That wasn't a ground loop — that was a faulty amp with a missing safety earth. There was no loop, because there was no earth at all until touching the radiator provided the missing link. Had you sat on the radiator and then picked up the guitar, you might well have been playing Emaj7 on a harp on a fluffy white cloud by now!

This is a distressingly common and life-threatening situation often caused by guitarists (or their so called 'technicians') in a futile effort to stop audible hums.

A ground loop is different — it occurs when there is more than one ground path between two items of equipment. Usually, one path is the screen of an audio cable connecting the two pieces of equipment and the other path is via their chassis safety earths in the mains plugs. Inside the equipment, the audio screen earth is often linked directly to the chassis earth, hence the possibility of a loop. If the two bits of equipment are plugged into the same mains socket, their chassis safety earths are effectively tied together at the same potential, and so there is unlikely to be any circulating ground current, despite the apparent ground loop. However, if one item is plugged into a different mains socket, its chassis safety earth might be grounded some way away from the other equipment's earth, and there can be a small difference in potential voltage between them. Silly as it sounds, earth is not the same everywhere. The potential voltage difference between their two chassis earths can cause a small current to flow, and since the earth provides a reference for the audio electronics, that flowing current causes the earth reference voltage point to vary slightly. This can be heard, usually as a low-level hum or buzz.

Ideally, the solution is to make sure that everything is earthed at one central point, so that everything shares the same common earth reference point. The easiest way to do that is to plug everything into a star arrangement of plug-boards fed from a single socket (assuming suitable power capacity). If that can't be done, the safest solution is to break the loop by isolating the audio cable screens at one end. The cable is still screened, but there is no longer any possibility of a loop, so the hum currents can't flow around it. Inserting transformers in the signal path can also break the loop, and this solution is common in outside broadcast and live sound rigs. DI boxes feature transformers for this purpose too.
Q. What is a Ground (Earth) Loop?
Problems arise when uninformed people decide to break the loop by removing the safety earth in the mains plug instead. This does break the loop, obviously, so any related hum will disappear. But it also means that the equipment is no longer earthed, and thus any fault that occurs in the equipment is now life-threatening! Sometimes you don't even need a fault to cause dangerous problems, though. Most equipment has filtering on the mains input to stop mains-borne noise getting in (or out). If you disconnect the mains earth in the plug, the nature of that filtering is such that the (previously earthed) chassis — and everything connected to it — 'floats' up to half mains voltage (making it about 115V in the UK). This means everything that should be safely earthed — all the exposed metalwork, including guitar strings — now carries a life-threatening voltage.

Going back to your guitar incident, the strings on the guitar are supposed to be earthed through the guitar lead to the socket on the amp. That, in turn, is usually connected to the amp's chassis earth, and thus through the mains plug to the mains safety earth. Metal radiators are connected to the mains safety earth point too, as is all house plumbing.

So if you have a guitar amp with the safety earth disconnected in the plug, the chassis is likely to rise to 115V, and everything that should be at 0V (earth) is now cooking with gas! Rest the guitar strings on anything that really is earthed (the radiator in your example, but a mic stand holding a mic that is earthed through its cable to a mixer is another very common alternative), and you have mains power now flowing directly through the equipment to find a real ground. This is almost certainly going to severely damage or destroy the amp, the guitar and — if you get yourself between the two — you as well! The classic way for budding pop stars to die is because of an amp with the mains safety earth disconnected. They rest one hand on their guitar strings and reach up with the other to hold a properly earthed mic. That leaves them with 115V effectively straight across their heart. Sweaty hands make very good conductors and it only takes a few milliamps of current flow to stop the heart. Musicians are electrocuted in this way every year, and while I'm all in favour of Darwinian evolution, sometimes the good guys get caught out too.
The lesson is that you should never remove the earth from a mains plug. If you have hum problems, break the screens on the audio cables or use isolating transformers in DI boxes.

Published August 2006