Digital interfaces allow scientific cameras (and other imaging or microscopy hardware) to communicate with computers, typically for data transfer so that images acquired during experimentation can be stored for analysis and processing.
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With camera hardware increasing in speed and field of view (FOV), experiments can produce large amounts of data, especially when looking at multiple dimensions such as time-lapse, XY scanning, Z-stacking for 3D imaging, multichannel imaging, and more. This means it is vital to know the pros and cons of certain digital interfaces, in order to match flexibility, compatibility, and data transfer rate.
Teledyne Photometrics CCD and CMOS cameras typically use either universal serial bus (USB) or external peripheral component interconnect express (PCIe) cables, both of which are explored in this article.
USB needs no introduction, as it has become an industry-standard connector for storage, power, peripherals, and data transfer. From the launch of USB as a platform in , there have been a number of different generations and iterations of USB hardware, which have changed names over the years and been subject to some re-branding. This section covers some generational changes in USB while demystifying the nomenclature.
From to there have been a number of different USB generations, as outlined below:
Figure 1: USB 2.0 vs 3.0. The USB 2.0 is typically white and features the standard USB symbol.
Each new generation introduced greater speed and transfer rates along with more efficient power usage. Rather than move onto a new generation, the next upgrade to USB came in the form of specification USB 3.1 (introduced July , max speed 10 Gbit/s), which featured twice the speed as USB 3.0. Due to the success of USB 3.1, USB 3.0 was retroactively renamed, as seen in Table 1.
Table 1: The different naming systems for USB digital interfaces
Original name New name USB branding Max speed USB 3.0 USB 3.1 Gen 1 SuperSpeed USB 5 Gbit/s USB 3.1 USB 3.1 Gen 2 SuperSpeed USB 10Gbps 10 Gbit/s
Following the release of USB 3.2 in Sep , this naming scheme was again changed, from USB 3.1 Gen X to USB 3.2 Gen X. Overall, this means that USB 3.2 Gen 2 refers to what was originally USB 3.1, a USB system with a maximum transfer rate of 10 Gbit/s.
At Teledyne Photometrics we follow the latest trends in USB technology, and our cameras typically use the fastest USB data cables currently available, with newer cameras like the Prime BSI Express and Kinetix using USB 3.2 Gen 2, a USB C type reversible cable for easy usage.
A computer motherboard typically contains several expansion slots for modular peripherals (such as RAM, a GPU, sound card, or additional digital interface) that can be swapped in and out, these are known as PCI Express slots.
However, PCIe can also refer to the external PCIe cable, a multi-purpose interface that has been widely adopted across the computing industry for the high data transfer rate, as seen in Fig. 2.
Figure 2: PCIe external cables. The left image shows a standard gen 3 PCIe cable, connectors at both ends. Centre image shows
PCIe currently has six iterations with the data transfer rate doubling each time, resulting in large data transfer rates, suitable for experiments that produce gigabytes to terabytes of information with each imaging session.
The overall maximum speed of a scientific camera is related to how much data the camera sensor can output, but if the camera can output data faster than the interface is capable of transferring it, it creates a bottleneck in the imaging system. As a general rule, each imaging system is only as fast as the slowest component, which means that researchers using imaging systems that need to operate at high throughput or high speed, need the more efficient and fastest data transfer technologies.
Historically, scientific cameras have used a variety of technologies to interface with computers and digital storage systems, from frame grabbers (expansion cards that captured individual frames) to Thunderbolt and GigE connections to allow the use of the full speed of the camera to transfer to the computer. However, these technologies are bulky, often need additional cards for the PC to have the correct interfaces, and can be difficult to use as in some cases the camera will need to load before the PC for the interface to function. This makes USB an attractive alternative as it is easy to use, flexible, and ubiquitous across PCs.
However, USB has a lower data rate than some of these alternatives, especially PCIe, as outlined in Fig.3 which demonstrates the differences between different generations of USB and PCIe over time.
Figure 3: USB vs PCIe data rate across generations. USB typically has a lower transfer
Teledyne Photometrics sCMOS cameras typically offer either a USB or a PCIe version, with more recent cameras such as the Kinetix offering both in one system, with the PCIe setup delivering greater speeds and framerates due to the ability to push more data down the cable during high-speed acquisition.
Overall, USB is a flexible and ubiquitous option that researchers already have access to, and recent versions can achieve 10 Gbps speeds that are suitable for applications such as cell documentation, or other imaging experiments that don't require high speeds, either due to a lack of dynamic motion in the sample, or the need for a long exposure due to a low signal level, which also limits speed. If speed is a necessity and the imaging computer has a free PCIe slot, PCIe external interfaces are more suitable due to the significantly higher data rate and ability to run high speed cameras such as the Kinetix at full speeds.
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by Pat Brown
Are you new to digital audio? Are you confused by the landscape of formats that have emerged in the marketplace? Following is a short expose that may shine some light on this potentially confusing topic.
Its all about inputs and outputs (I/O). How do I get an audio signal from one to the other? The ongoing evolution of professional audio has produced a number of viable digital interfaces to complement legacy analog I/O practices. The choices may seem confusing at first, but when you break them down the strengths and weakness of each become apparent.
In this overview, I will start with analog since it is familiar to most readers and serves as a reference for the discussion of digital formats. I will focus on professional interfaces only. While similar in many ways to consumer I/O, professional are more robust against electromagnetic interference (EMI) and allow much longer cables both requisites for large sound systems.
Figure 1 An analog interface is point-to-point, with no exact requirements with regard to cable type, connector type, and cable.
A professional analog interface is a point-to-point connection between an output and an input (Fig. 1). It is electrically balanced, which provides strong immunity to EMI. Cabling is shielded twisted-pair. The interface is impedance mis-matched, where a low output impedance (typically < 200 ohms, or Ω) drives a high input impedance (typically > Ω). The impedance mismatch simplifies the interface by (usually) eliminating the need to consider specific impedance values. For example, a 100 ohm output would produce the exact same signal level into any high impedance input (10kΩ, 20kΩ, 30kΩ, etc.). An output simply connects to an input end of story. It is often permissible to passively split an analog output to drive several inputs. It is not permissible to Y multiple outputs together.
The signal is in the form of a time-varying analog voltage that can span a level range of over 100 dB. Signals are classified by the magnitude of this voltage (e.g. mic level, line level, loudspeaker level). The signal flows in one direction only from output to input. Cable lengths are limited by cable capacitance, and can approach 300 m ( ft) in some applications.
Analog connectors are classified by the number of electrical contacts. The impedance of cables and connectors is not a consideration, since analog audio signals are low in frequency in terms of the electromagnetic spectrum (Fig. 2).
Figure 2 Analog audio (sometimes called baseband audio) is below 100 kHz in terms of spectral content. This is low frequency in terms of the entire electromagnetic spectrum.
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The professional audio industry has standardized several point-to-point digital audio formats. These are designed to largely insulate the user from the complex inner workings of the interface and the complexity of the data. The most common are AES3 (USA) and AES-EBU (Europe). Ill use AESx to refer to both. They are mostly identical except for a few details.
Figure 3 A digital audio interface has more exact requirements than an analog interface. This includes an (approximate) impedance match, and there are less connector options.
Like analog, AESx is a one-way connection between an output and an input. AESx was designed to allow the use of balanced analog cabling and connectors, most often an XLR male output driving and XLR female input via a shielded twisted-pair cable. For short runs (< 30 m, or 100 ft), one may use the same cable as for a balanced analog interface. Low-capacitance cable designed specifically for digital I/O can extend the cable length to 100 m (300 ft). As always with cabling, your mileage may vary depending on the specifics of your application. Other cabling options exist including coax (AES3id essentially a video interface) and category cabling (e.g. CAT5e). AESx can also be deployed using space-saving DB25 connectors on multi-pair cable (up to 16 channels). The exact implementation is product specific, and there are passive methods for converting between each. The manufacturer determines the specifics based on the target market for the product.
An important difference between analog and AESx is that an AESx connection carries two audio channels over a single twisted-pair. There is nothing special about the XLR connectors, and some products allow the user to select between analog and digital using the same input or output connector, saving space on the chassis (Fig. 4). The high compatibility with analog I/O cabling and connectors is a major strength of AESx.
Figure 4 The input connectors for a four-input AESx processor. Note that for XLRs are required for analog, but only two are required for four channels of AESx (two channels on each connector). There is a switch to select between analog and AES (courtesy Marani).
The objective of digitization of the analog signal is lossless encoding. This requires a sample rate greater than 2 times the analog signal bandwidth, along with a dynamic range that approaches 100 dB. So, the main attributes of the data are the bit depth and sample rate. Most devices default to 24/48 which means 24 bit words flowing at a 48 kHz sample rate. Greater sample rates are possible, and are sometimes used for special applications.
The sample rate, bit depth, and number of channels can be multiplied to yield the data rate. This gives us a simpler, one number way to describe the digital audio resolution. Figure 5 shows the minimum data rate for one channel of full-range audio. A strength of digital audio in general is that the data rate can be reduced using lossy or lossless compression schemes. AESx signals are usually not compressed to reduce the data rate, since the minimum requirements for full resolution are easily met by the current technology.
Figure 5 Sample rate and bit depth can be expressed as a data rate for the digital bit stream, a sort of conveyor belt for the ones and zeros that make up the digital signal. AESx digital audio consists of two channels, with a data rate of about 6 Mbps.
The bit stream contains the audio samples (or payload) along with metadata that carries information about the signal required for decoding. This protocol must be adhered to by both the output and input circuits, or no audio will flow.
Since the data rate approaches 10 MHz (a 1.5 MHz fundamental plus odd harmonics), the interface must be impedance-matched (110 Ω) to prevent degradation of the signal traveling down the cable by reflections and standing waves. Like all impedance-matched topologies, AESx is a one-to-one connection one output drives one input.
Multi-channel versions include AES10 (MADI) and AES50 (HRMAI). While based on AESx, the details regarding clocking are quite different. These multi-channel interfaces are popular for connecting digital mixers to their respective stage boxes.
All digital signals require a clock signal to keep multiple components in synchronization. AESx has the clock signal embedded in the data stream, so a dedicate word clock connection is often not needed for simple systems. AESx components often have word clock I/O for more complex systems.
The output of a digital component is always latent relative to the input. There is an unavoidable delay. Latency is cumulative, and system designers have a latency budget that must be observed to avoid timing issues.
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Once the analog audio signal is digitized, its data. Technology has provided some very efficient means of data transport between computers, the most widespread of which is the Ethernet network. Audio-over-Ethernet (AoE) exploits the low cost and ubiquity of data networks to transport audio data. While AESx and its siblings are audio industry-specific, AoE utilizes technology from the Information Technology (IT) industry as a means of audio transport.
Figure 6 AoE requires the connection of each component to a network switch. The I/O connections are made using a configuration program (e.g. Dante Controller or Cobranet Disco).
The analog waveform is first sampled, and then packet-ized. A packet consists of a few audio samples and some additional meta data necessary to traverse the data network. The AoE protocol (e.g. Cobranet, Dante, Q-Sys, Ravenna there are others) assures that the data gets on and off of the network to produce a contiguous waveform at the receiving end. High speed networks (e.g. BaseT or Gigabit) can transport hundreds of channels. Unlike the one-way street AESx formats, a single cable can carry signals in both directions.
Regarding I/O, its a data network and the rules are rigid and established by the IT industry. The user is largely insulated from the complex inner workings of the network, and deployment in smaller systems is relatively plug-and-play.
Audio practitioners must acquire IT skills to deploy large AoE networks. Ideally, an audio-only data network is established using dedicated network switches. Increasingly, AoE may reside on the venues backbone with other data traffic such as , web browsing, point-of-sale, etc. the so-called converged network. On a converged network, the audio packets must have higher priority than other data traffic. This is called Quality of Service (QoS). A chunk of a large network can be reserved for audio data through use of a Virtual Local Area Network (VLAN). A VLAN requires a more sophisticated managed switch than a simple, dedicated network.
Due to the simple wiring, quantity of channels, and low-cost network hardware, AoE has become a very popular digital audio transport. A major difference between AoE and AESx (and its siblings) is that AESx, like analog, is point-to-point, requiring a direct connection between the output and input. AoE is a network, so signals can be routed between any network devices, regardless of where they are connected to the network. This provides extreme versatility regarding signal routing for larger venues, campuses, studios, etc.
The audio practitioner must learn the specifics of the AoE flavor that they are using. This includes running a control application on a PC to route audio signals, and configuring network switches for the required QoS.
There are competing AoE formats. As of this writing, Audinates Dante enjoys the most widespread use. This can (and probably will) change with time, and competing formats such as Audio Video Bridging (AVB) (now referred to as Time Sensitive Networking (TSN)), will emerge as alternatives. AES67 is an initiative to produce inter-operability between the various AoE formats, so it may eventually be possible intermix AoE formats.
It is important to point out that AoE is just a transport. The quality of the digital audio is determined by the sample rate and bit depth. As such, all AoE formats should sound the same.
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One way to frame the differences between the formats is in regard to the requirement for exactness of the interface variables. This, in turn, is based on where the signal spectrum falls in the overall electromagnetic spectrum. As with most things audio, it is a gradient, with no clear right or wrong answer. Since audio by definition is low in frequency, analog audio interfaces are fairly permissive with regard to the interface details. One can sort of get away with murder and still get audio. Many wire and connector types will work but it is good practice to use twisted-pair cabling, ideally with a shield. Cable testing is basically a simple continuity test. The signal pair can be bridged with a butt set to detect the signal (Figure 7).
Figure 7 A simple butt set is all that is required for analog troubleshooting.
Digital I/O moves further up the spectrum. The interface details become more black and white. For example, analog audio simply requires a low impedance output be connected to a high impedance input. All digital I/O formats require an impedance match, but AESx is more permissive than AoE. This explains why we dont need special XLR connectors for AESx signals. Its also why Euroblock and DB25 connectors can work. Even analog cabling can work at short distances. Cable testing is often performed by the audio gear itself, and whether the AESx input can lock to the AESx output. Special testers are available that allow the lock signal to be detected, and the audio decoded and listened to (Fig. 8).
Figure 8 A specialty tester for AESx interfaces (courtesy Whirlwind).
At the highest frequencies (AoE on a gigabit network) the interface details are so stringent that they cant be left to chance. The interface, cables, and connectors have an exact specification and no decisions are left to the end user. While continuity testers can work at the most basic level, the testing can (and should) be much more sophisticated. Along with cable testing there is a need to verify the system bandwidth, component addressing, Power-over-Ethernet (PoE) requirements, and more. Instrumentation for troubleshooting can be PC-based or stand-alone (Fig. 9).
Figure 9 A stand-alone tester for network troubleshooting (courtesy Fluke).
Figure 10 The requirement for precision in interfacing depends on the spectral content of the signal. The higher the data rate, the more the details matter.
If you read my stuff then you knew this was coming. Heres a way to describe the various interface methods to the lay person.
Analog is like driving a jeep on a one-way dirt road. In spite of the bumps, the vehicle can weave all over the place and still get to its destination intact. AESx is like a Formula One car on an oval track. The higher speed requires a smoother surface and some boundaries. AoE is like a bullet train. There is an exact path and the margin for error is razor thin. At those speeds, any small discontinuity can have disastrous consequences.
This also explains why there are so many poorly performing analog systems in use. Impaired analog interfaces can still pass audio and no one may even realize they are impaired unless there is a direct comparison to a more ideal interface. So, as we move up in the spectrum from analog, to AESx digital, to AoE the interfacing details become more exact and there is less room to bend the rules. What we get in exchange for increased complexity are more channels and greater versatility valuable benefits for many system types.
Digital audio has not obsolesced analog, and AoE has not obsolesced AESx and its siblings. All have their place. Analog still remains the standard by which to judge the fidelity of a digital system. Sometimes the simplest approach is the best, and sometimes it isnt. pb
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