Analog vs. Digital Cameras – What is Being Used in Machine Vision?

Recently this question was posed on Machine Vision Online, and several most interesting responses were given.  This article combines the answer to that question with the notes taken on a session given at the recent AIA Business Conference titled 'The Pros and Cons of Firewire, Camera Link, Gigabit Ethernet and USB 2.0.'  Those responding to the question included:

Terry Guy - Eastman Kodak
Christophe Robinet  - Atmel
Charles D. Gilbert - Cohu Electronics Division
Toshi Hori - Pulnix
Jerry Fife - Sony

Christophe Robinet: 'In the future I believe we will see three dominant interfaces:

1. USB1/2: Low-mid range area scan cameras; mostly digital still and VGA video (webcam>camcorders)
2. 1394a/b: mid-high range area scan cameras; camcorders and industrial/scientific
3. CameraLink: Mid-high range line and area scan cameras; mostly industrial Legacy (analog and digital) will continue to exist for some time and specialty interfaces will be developed and offered to address special needs.

I don't see any one interface able to address all the needs of all the markets. The part of digital is slowly increasing, but as far as I know, the worldwide 2002 market still in favored analog (mainly based on AIA results, and discussions with competitors).  As for Atmel, we focus on LVDS and CameraLink. A question remains: what will analog systems be replaced by?  Firewire, USB, Ethernet or other LVDS/CameraLink interfaces?  I do not have the answer.'

Jerry Fife agrees with Christophe 'that most of the industrial cameras are still analog. The AIA Market Report for North America certainly indicates that most industrial cameras are analog because they are used in a large number of relatively 'simple' imaging areas that is easily addressed by VGA resolution with 30 frames/sec speeds and frame grabbers.  The higher end industrial and scientific market is predominately digital. I believe that a large percentage of the digital industrial market is line scan; over 50% of the CameraLink cameras sold currently are line scan.  In the scientific world, almost all of the cameras are area scan and CameraLink represents a small percentage of cameras sold.'

Toshi Hori also agrees that 'analog camera (even mostly TV-format) is still major part in quantity base. Due to the slowdown of industry, new equipments are not in high volume sales and end users like semiconductor companies only buy current systems on as needed base. So just shipping quantity is not indication of the industrial trend. However, I have seen new specifications of new equipment models, which are quite different. It is very obvious large portion of high volume machine vision cameras are going to progressive scan from TV-format even though both are analog. Then next trend is going to digital (in various format including IEEE-1394, CameraLink, LVDS, USB-2.0). 60% of our new business for area scan cameras today is digital and CameraLink share is quickly increasing (20% last year and 50% this year in shipment quantity). Most of new inquiries for high-resolution cameras are CameraLink. Virtually all of multi-tap output cameras are CameraLink. So realty of currently-shipped camera and new spec-d in may have at least two years of time gap.'

Charles Gilbert:  'While the majority of new standard products developed by Cohu in the last 12 months have been digital (6 CameraLink, 1 IEEE 1394A), our sales continue to be dominated by analog output cameras.  I think Toshi Hori makes an excellent point, that the current market status has most volume OEM's in maintenance mode.  The plan for many upgrades have been written during the downturn, but the execution of the upgrade will most likely occur when there are sales to support it. When the market picks back up in a more consistent nature, we as an industry will see more digital output opportunities. However, for price and simplicity, analog output cameras will always own a good chunk of total camera units sold.

As far as what style of digital output cameras will dominate (USB 2.0, IEEE 1394A/B, CameraLink, Ethernet), who knows?  We obviously think CameraLink is important to have in our arsenal.  We have some customers demanding IEEE 1394A, yet others, believe USB 2.0 is the future and for them it might very well be.  For our high-end security market place, Ethernet is king.  We are absolutely sure of only a few things: 

1. Standard LVDS digital outputs with their infinite cable combinations will eventually die away.
2. We need to learn through study and development experience about the other outputs because theory and educational articles written by university professors do not produce usable video data.  Design, build, and test the camera in an application and that will answer at least a miniscule part of the enormous question at hand.
3. We'll never convince this industry's customer base to want the same thing.  There might be trends in niches of the industry, but camera manufacturers will have to provide options in order to compete in multiple facets of our customer base.'

Terry Guy: 'The majority of cameras sold for machine vision application's are VGA format with analog outputs. Digital outputs have been primarily used with High-Resolution and Line scan cameras. While the adoption rate of High-Resolution cameras in machine vision has been slow, I think we will see a significant increase once we come out of this Semiconductor/Electronics market slump. As Toshi says, I believe their are new designs waiting to be released once the Semiconductor and Electronic markets turn around. As for the future, which interface will win: USB, Firewire, GigaBit Ethernet, or Camera Link, my opinion is that application's requiring real time synchronous operation with the Vision System will choose CameraLink. I think these will be primarily OEM equipment builders using MV in their systems. Applications that are off-line and do not need time critical triggering and image capture could benefit from Firewire or USB from a reduced system cost point -of-view. Which one wins, Firewire, USB, or Gigabit Ethernet I cannot guess at yet. My opinion here is that the interface that gets Microsoft support, programming tools and Windows support, will win.'

The following review of different camera interface standards comes from the recent AIA Business Conference held in Orlando, Florida.

CameraLink Overview - Jason Mulliner
CameraLink is a standard out of the AIA standards committed providing a serial connection for digital cameras based on National Semiconductors Channel Link chips. It includes:

• 28 pin connector, signal serialized 7:1
• image and timing signals
• 4 signals for camera control
• 2 signals - RS 232
• base performance with one connector of 255 MB/s; 383 MB/s with two connectors and full 680 MB/s with 2 connectors

The benefits include:

• Lower cost of vision systems
• Standard connector and cables
• Smaller flexible cables
• Faster to first image
• Flexibility
  • High data transmission rates
  • Provision for area and line scan
• Active committee with camera, frame grabber and connector companies working together on standardization - now 38 companies

Disadvantages cited:

• Limited standard cable length - but fiber optic extenders have emerged
• Limited product availability - but growing
• Not plug and play - being addressed by the committee

The committee is now focusing on:

• Ease of use
• Serial enhancements
• Smaller connectors
• Well define data configurations

Firewire (IEEE-1394) - Jerry Fife
Cited the key features as:

• High bandwidth - currently 400 Mbits/sec (40 Mbytes/sec) with future at 800 and 1.6Gb/s and beyond anticipated
• Deterministic - isochronous transfer (guaranteed bandwidth: time critical data (video, audio, etc.); one to all; broadcast; asynchronous transfer (guaranteed delivery: content critical data (command, files, etc.); one to one; confirmed messaging
• Flexible topology - tree or star network topology: up to 63 devices per bus; peer-to-peer (multiple CPUs)
• Many devices per network - hot pluggable, plug and play, automatic reconfiguration (no requirement to reboot; no need to ID switches or addresses)
• Small connector and cable - 4 pin (no power); 6 pin original; 6 pin latching; 6 pin latching hardened; wire pairs: data in, data out, power

Suggested reasons for using a digital camera include: reduce or simplify cabling (lower system cost, network versus many boards and cables); remote control of camera setting; higher resolution, non-video format; flexibility of high resolution or high rate; work with smaller, slotless computers.

Provided the following comparison table of connection methods:

Suggested the following are myths surrounding IEEE-1394:

• Need longer cables
• Need more bandwidth for multiple cameras
• Need to trigger fast
• Limited choice of suppliers and software
  • Good range of cameras available from many vendors
  • Many support components are available
  • Lower cost no network configuration
  • Performance for many machine vision applications 

Ethernet - John Vieth
Defined camera interface as a means to connect a camera to an image processor over some distance to exchange image data and control signals comprised of: a detachable physical interconnection and a signaling protocol. Offered the following as primary considerations:

• Speed and latency of data transfer - interface is not the bottleneck
• Cost of implementation
• Compatibility/interoperability
• Readily available components - off-the-shelf
• Ease of installation and maintenance - plug and play
• Fault tolerance/noise immunity - low bit error rate over long distances
• Reliability - work over various operational situations

Described Gigabit Ethernet as:

• Base standard that is over 20 years old - IEEE 802.3ab standard 1000BASE-T
• PAM5 signal encoding (5 level Pulse Amplitude Modulation)
• 1 Gigabit per second over wire
• 4 unshielded twisted pair Category-5 wiring
• up to 100 meters
• 8-pin RJ-R5 connectors
• As a camera interface Ethernet 
• Can carry 80 million 10 bit pixels per second of camera output
• Readily available cable and connectors
• Relatively long distance span - 100 meters
• Proven noise immunity and reliability
• Off-the-shelf PC adapter cards - Dell is shipping Pentium 4 with board already inside
• Commodity pricing of components as leveraging PC volumes

Compared to other interfaces:

Advantages

• Greatest distance span
• Simplest cabling - standard
• Slightly higher speed than USB, Firewire
• No unique PC interface hardware
• Potential for lowest material cost

Disadvantages

• Lower speed than CameraLink (insufficient for higher speed cameras)
• Lacks the standard API and PnP support of USB and Firewire
• Not plug and play - need to install driver