sampled instruments
2.4 Using a line input/instrument input
Instead of using a microphone to capture your sound source, you can sometimes use a direct input, such as a line input or an instrument input. Although the actual input on either a hardware sampler or on a preamplifi er for line and instru- ment inputs might look the same, they are different. The primary sound sources you might capture using these methods include synthesizers, keyboards, electric guitars, bass guitars, or any source that comes from a computer or other hardware sampler. Guitars, bass guitars, keyboards, and synthesizers are typically recorded through an instru- ment input, but some synthesizers, keyboards, and all com- puters/samplers will utilize the line input. The difference is in the electronic impedance common to each of these sources.
There are three common electrical signal levels used in audio electronics. These are colloquially referred to as microphone
level, instrument level, and line level. Each signal varies
in strength and results in different electrical impedance requirements. The impedances for each type vary quite a bit. An electric guitar might feed into an impedance of 1,000,000 ohms (1M). A low-impedance microphone might feed into 250 ohms. A line level input on a console might accept around 10,000 to 50,000 ohms (10–50 k). Without going into much detail, this information is designed to reinforce the differences between each of the three signal levels. A microphone preamp amplifi es a microphone signal many times to bring it to line level. A preamp that accepts instru- ment input levels also amplifi es the instrument signal to line level. Mixing consoles and other audio equipment operate at line level, so the objective is to use appropriate preamplifi ca- tion to bring all sources into the signal path at this estab- lished line level.
Audio Sampling
Most hardware samplers have either a microphone preampli- fi er or a line-level input. Some have instrument inputs as well. If you want to connect an instrument-level output to a sampler but the interface has only a microphone input, then you can use a direct box, or DI (direct injection). A direct box is a transformer that takes a line-level signal and steps it down closer to a microphone-level signal (Figure 2.7). You can then hook the direct box into the microphone preampli- fi er. A direct box also converts an unbalanced signal to a bal- anced signal. Unbalanced and balanced signals are discussed later in this chapter (see Section 2.11).
If you are using a software sampler in your computer, then the input choices depend on the specifi c audio interface. Some audio interfaces, especially those designed with a non- professional audience in mind, have a variety of different input options. If you operate in a home studio, then it is great to have a single interface that has line input, instrument input, and microphone input. It is also nice when using a portable interface because it is more effi cient to have a single piece of gear.
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2.5 Using microphone preamplifi ers
Microphone preamps are a tricky part of the audio chain. The only way you can really tell if you have one that is good (or bad) is to compare it to other preamps, although sometimes you can detect obvious fl aws. Some people claim they can identify the traits of various preamps and hear how preamps “color” the sound differently. Still, when capturing a variety of sound sources using a variety of microphones, it can be diffi cult to predict the success of the preamp. Recording studios often like to have various preamps on hand because the engineers feel that certain sound sources require different microphones and preamps.
A preamp, as stated earlier, amplifi es the microphone signal to a line-level signal. This is required for standard processing, including preparation for conversion from analog to digital. The electronics used in the preamp determine the sound of the preamp. Some preamps use solid-state amplifi cation; others use tubes for amplifi cation. Some preamps use trans- formers while others do not. You may well be satisfi ed to use the preamp built into your sampler or your audio interface. But if you choose to purchase even one additional preamp, it can be hard to decide which one to buy. Preamps come in a wide range of prices and styles. You can easily spend thou- sands of dollars for a single preamp. The best approach is to try out as many as possible. Don’t simply buy the one you can afford. Sometimes a specifi c preamp style just isn’t right for the recording you wish to do. Sometimes the most expen- sive preamp isn’t what you are looking for. You might con- sider buying a very clean and transparent preamp for general use, because such a preamp often works well on a variety of sound sources.
When testing out potential preamps, here are some questions to ask yourself:
1. Does the preamp change how the natural source sounds? How?
2. Can the gain of the preamp be pushed very high without a lot of distortion?
Audio Sampling
3. What other features does the preamp have? (equalizer, com- pression, digital outputs)
4. How expensive is the preamp?
When testing out the preamp, use a single microphone so you can make accurate comparisons. Consider using a measure- ment microphone, such as the Earthworks M30, which has a fl at frequency response. This helps the preamp quality and character shine through.
2.6 Using analog-to-digital converters
Once your signal is inserted into a preamp from a micro- phone, an instrument input, or a line-level input, you have to take the analog signal and convert it to digital. Hardware samplers and audio interfaces have analog- to-digital converters built in. The question is not whether you have access to an analog-to-digital converter, but how good it is. Some external preamps have great analog- to-digital converters, but you can also fi nd stand-alone converters that do nothing but analog-to-digital conversion. In listening tests, it is possible to hear the difference between poor and great converters, but the real challenge is not in the choice of converters but in how to hook them together.
With a hardware sampler, you can only use an external analog-to-digital converter if there are digital inputs. The type of digital input is important because there are several different digital formats and they are not compatible with one another. The most commonly used digital formats are S/PDIF, ADAT optical, and AES3 (these are discussed later in this chapter; see Sections 2.9 and 2.10). It is important to understand the different types of digital signals.
Likewise, it is critical to understand how a word clock func- tions in a digital audio system. Every audio system that deals with digital information needs to have a master clock that regulates the movement of all digital information and syn- chronization between different pieces of equipment. If you have more than one master clock, there can be “clicks and
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pops” in your audio signal. Sometimes the equipment won’t even play audio.
The master clock can either be the internal clock of one piece of equipment (every piece of equipment that uses digital signals has one), or it can be an external box designed to be a master clock. To connect two clocks together without an external master clock, you can use the digital cable you are already using. AES3, S/PDIF, and most other digital signals have clocking signals embedded with the audio data. You only need to tell the equipment who is the master and who is the slave, and then it will work. This is sometimes a menu choice or a switch on the back. See the equipment’s specifi c instructions for more information.
The other method for connecting a master clock to a slave is through word clock input and output ports. These typically use BNC connectors with 75-ohm cables. Depending on the system, you will likely run cables from the master to a slave and then from that slave to the next until all pieces are connected. The last piece of gear typically connects back to the master, creating a loop. A second option is to have a master clock with multiple outputs. Each output connects directly to each piece of equipment. In this case, each slave needs to be terminated using a 75-ohm word clock termina- tion plug.
In addition to sending clock information, most gear has to be set to master or slave mode. While you can purchase a piece of gear that is a dedicated clock, you can also use nearly any piece of digital gear. Not every piece of digital gear has a great clock (poor ones can create timing errors called jitter). If you are going to use an analog-to-digital converter instead of the hardware sampler’s converter, you have to make sure that the sampler or the converter is set to be the master, with all other devices set as slaves.
One large benefi t of using a separate analog-to-digital con- verter is that you can bypass all of the analog components on a hardware sampler. Instead of using a decent preamp and
Audio Sampling
decent converter, you can get really great components and preserve the quality by sending the data into a sampler using a digital format. Digital information is able to retain a higher level of accuracy when transferring signals from one place to another, while analog transfers are more susceptible to dis- ruptive noise and interference.
2.7 Using audio interfaces
An audio interface is a basic requirement when using a soft- ware sampler. Most computers have some sort of microphone input and speaker output, but using the built-in hardware is not a good fi rst choice. Many onboard microphone inputs use cheap components and are very noisy. An external audio interface can be used to bypass the built-in hardware, and this creates an environment with much higher-quality possi- bilities. The different types of interfaces use expansion cards, USB (universal serial bus) ports, and FireWire (IEEE 1394) ports. There are also other miscellaneous connection proto- cols, but these three are the most common.
2.7.1 Expansion cards
Expansion cards are a standard way to connect an audio interface. Some of the most powerful audio interfaces use expansion cards that are installed inside a computer. Some expansion cards even have additional computing power available in the form of DSP (digital signal processing) chips. The DSP removes some of the processing from the computer, freeing up resources for use in other places. Expansion cards can have a variety of digital and analog inputs/outputs, but one of the best options is to use a card that provides many digital ins and outs. The quality of the audio is then reliant on the quality of the external analog-to-digital converters. This means that the expansion card acts as a gateway into the computer, but you do need separate preamps and converters. If money is not an issue, this is an ideal way to go. But there are some good expansion card interfaces that also have preamps and analog-to-digital converters.
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2.7.2 USB and FireWire
USB and FireWire ports are used as a means to connect audio interfaces to computers. There are multiple USB and FireWire types, including USB 1.0, USB 2.0, FireWire 400, and FireWire 800. The latter versions of each type are the newer versions, and these can transfer larger amounts of data in less time. When choosing an audio interface, you should be aware of two issues. The fi rst is compatibility, so you should research which interfaces are compatible with different computer hardware. Most manufacturers publish a list of equipment that has been tested with specifi c computers, including con- fi gurations that work and some that should be avoided. Just because your computer has a FireWire port doesn’t mean that every audio interface that connects using FireWire will work. Sometimes only a certain manufacturer’s FireWire port will work; likewise for USB 1.0 and 2.0. Another thing to watch out for is how an interface is powered. Some USB and FireWire interfaces can be powered directly from the computer, but if you hook too many devices to the same port, there might be insuffi cient power.
2.7.3 Miscellaneous connections
Other methods of connecting audio equipment to a software sampler include mLAN, digital inputs, and wireless inputs. mLAN is a proprietary format that is available in some cases but is not yet widely used. mLAN can transfer a number of different audio channels, along with other information such as MIDI data and control data. It uses a computer network- ing protocol when transferring data and works fairly effi - ciently. Another way to get audio into your computer is through any existing digital inputs on the computer. More and more computers now come with a set of digital inputs and outputs, but this can be a limiting factor because typi- cally there is only a stereo input and output. Wireless con- nectivity remains relatively undeveloped due to limited wireless bandwidth. However, it will not be long before everything is connected wirelessly. Imagine a world without
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cables! While not unrealistic, this technology has yet to be fully developed.
2.7.4 Monitoring
The fi nal step is the playback system (monitoring). Do not cut corners here. Most computers have poor audio playback. You will be making some very critical listening judgments during sound sampling and editing, so it is advisable to invest in a quality listening environment.