Haeco-CSG
The Haeco-CSG or Holzer Audio Engineering-Compatible Stereo Generator system is an analog electronic device and method developed by Howard Holzer. His company, Holzer Audio Engineering, developed the system in the 1960s during the years of transition from monaural to stereophonic popular music recording. The process was used primarily from about 1968 until 1970 but still exists on a significant number of recordings made during the time.
Reasons for Using the System
The idea behind Haeco-CSG was to create stereophonic vinyl LP records that when played on monaural equipment would allow the 2 channel stereo mix to automatically "fold-down" properly to a single monaural channel.
The reason for the process is the compatibility issue between stereophonic and monaural recordings: information which is identical on both the left and right channels of a stereophonic mix sounds too loud when played back on monaural AM and FM radio stations and phonographs. When the left and right channels are summed together, any musical parts that are common to both channels combine to be 3 decibels louder than they are in the same mix when played in stereo. Vocals, solo instruments and bass lines are often mixed equally to both stereo channels — these sounds tend to be too loud when heard in mono.
Due to technical issues in the record playback process the practice of creating separate mono and stereo mixes of the same recording was common during the 1960s. This also required manufacturing and distributing mono and stereo versions of the same album title. The Haeco-CSG system appeared to be an attractive option for record companies and retailers by allowing them to cut costs. The engineers would only have to produce a single mix, the record companies would only have to manufacture and distribute one version, and vendors wouldn't have to waste space stocking two different products.
How the technology works
Haeco-CSG technology works on the basis of phase cancellation. When two waves that are not in phase are mixed, the resulting waveform has an attenuation in accordance to the degree of shift. For example, two waves which are 180 degrees out of phase will entirely cancel out when mixed together whereas two waves which are entirely in phase will double in amplitude. A difference in phase between 180 and 0 degrees results in a partial cancellation, which is the effect Haeco-CSG takes advantage of.
The system electrically rotated the waveform of the right channel by up to 120 degrees to control the buildup of center information during a simple mono downmix. It is not known how exactly the circuit accomplished this; it did however do this without slightly delaying one channel. The most common listing of 90 degrees out of phase corresponds to Haeco's own recommended[1] setting of the +3 dB build up, whereas no build-up would require a 120 degree offset. This setting was most commonly used because of its robustness against polarity reversal of audio interconnects down the chain, which can affect the resulting audio when downmixed to mono. Information that is not common to both channels is entirely unaffected as there is no offset phase wave to cancel with.
Negative effects
Generally speaking Haeco-CSG has a degrading effect on the performance of both stereo and mono sounds processed through the system. The effect can vary substantially from one recording to another depending on the characteristics of the original unprocessed sound. The system "blurs" the focus of lead vocals and other sounds mixed to the center of a stereo recording. As bass frequencies are centered on most recordings it also causes a partial loss of low frequency information, making the resulting sound somewhat "tinny." This negative effect can be heard on any stereo speaker system, but makes headphone listening particularly un-natural sounding. This is because the lead vocalist or performer's audio waveform would be attempting to partially cancel itself inside the listener's head, confusing the brain's audio positioning sense.
Due to complicated issues of phase and frequency it is difficult to predict exactly how the reduction from two channels to one will affect the sound of a particular instrument. Therefore mono sound from a true mono mix is always preferable to the use of an automatic process such as Haeco-CSG which converts stereo to mono.
Recordings Known
Atlantic Records took out a full page advertisement in the April 6, 1968 issue of Billboard Magazine to promote its adoption of the technique, calling it "CSG Stereo." Many A&M Records LP releases during the period including popular titles by Sérgio Mendes and Herb Alpert were released with this audio process starting in September 1968. Other record labels soon followed suit, and an estimated 10% of all stereophonic albums released during the late 1960s and early 1970s employed the system. Other labels known to have used the system include Warner Bros. Records and Reprise Records.
One of the biggest selling albums using the process is The Association's Greatest Hits, released in 1968. Most of the studio recordings on Wheels of Fire by Cream also used this process. Early copies from 1968 of Neil Young's self titled debut album used the system.
Use of Haeco-CSG in promotional recordings for radio
The original intention of using Haeco-CSG on commercial LP releases was rather short lived, however, use of the process continued well into the mid-1970s on promotional records sent to radio stations. Many commercial FM Rock stations did not transition from mono to stereo broadcasting until the mid to late 1970s. AM Pop music stations continued to broadcast in mono, as AM stereo broadcasting was not introduced until 1982 and did not become popular until the mid-1980s.
Many promotional singles and some commercial singles from the Warner/Reprise/Atlantic label group from this era had "CSG Mono Process" or "CSG Process" printed on the labels. Artists included Frank Sinatra, Gordon Lightfoot, James Taylor, Seals and Crofts. Warner subsidiary labels such as Atlantic issued a series of mono radio station promotional LPs by progressive rock artists circa 1968-1970 which are now rare collectibles. The series included titles by Led Zeppelin, Yes and many others. In 1979 the Warner distributed label Sire Records issued a promotional single of "Pop Muzik" by M which contains both short and long versions in CSG processed stereo. This may be the latest known recording to utilize the CSG stereo process.
Modern remastering without Haeco-CSG
Haeco-CSG can be applied during the mastering stage, near the end of the production chain for LP records. In such cases, the earliest analog master tapes for these recordings still exist without Haeco-CSG processing. Therefore, when these recordings employing the Haeco-CSG process are properly remastered for compact disc the effects of the process can and should be removed. Remastering without the effect usually requires the work of a well informed audio engineer who takes the extra time necessary to locate the correct master tape copies.
However the Haeco-CSG processing was often applied at the master tape mix session. This, in effect, makes it a permanent part of the stereo recording. However, the process can still be reversed through modern digital reprocessing. Unfortunately, many compact discs of these processed albums still are encoded with the system, causing the center channel blurring effect even on modern digital playback systems.
Digital reprocessing
Haeco-CSG processing can be reversed through digital audio workstation software by digitally re-rotating phase of the right channel back by the correct number of degrees.
Adobe Audition is able to remove the effect using the Graphic Panner tool (the Automatic Phase Correction tool is unable to accurately do this) by manually selecting the "Phase -90 degrees" preset. The "Auto Center Phase" and "Learn Phase" features will also work, but are not recommended. There are sometimes slight offsets caused by various mixing effects and, to a lesser extent, tape-head misalignment; studio reverb or naturally decaying reverb is a prime example. As a waveform decays in a large room, it naturally changes phase. In Auto Center phase, this is (generally) shown as the upper frequencies making a drastic change. Tape head misalignment (azimuth) also will cause a phase change in upper frequencies. One should be aware of this when attempting to remove Haeco processing and not use auto-phase options. Azimuth alignment adjustment tools can however be used AFTER Haeco has been removed.
The "Stereo Tool" plug-in used in conjunction with Winamp is also able to (mostly) reverse the effects of Haeco-CSG. To do this enable "Stereo Image Processing" option and adjust the "Angle" setting right to 90 degrees. By adjusting the Winamp plug-in settings under the "Preferences" tab to "out_disk" it is also possible to capture the reprocessed digital audio to a wave or mp3 file.
One can also use a simple channel mixer found in most workstations to do a simple correction. By having each channel contain 75% of itself and 25% of the other channel, the "blended" result will be mostly in phase. This, however, can cause the stereo separation to be diminished by a slight degree.
Use today
While the system is no longer in use anywhere today, the basic idea of shifting the phase to create a mono downmix can be applied today if one has a reason to do so. The "encoding" process is similar to the "decoding" process, just apply a 90 or 120 degree phase shift and then average the channels together in a channel mixer.
External links
- April 6, 1968 advertisement in Billboard magazine introducing the CSG process
References
- ↑ Audiorents.com. Library. Manuals. HAECO-CSG Instructions. Accessed February 9, 2009