HamCall (October 1991)

home *** CD-ROM | disk | FTP | other *** search

/ HamCall (October 1991) / HamCall (Whitehall Publishing)(1991).bin / bcast / miscbcst / cartmicr.txt < prev next >

Wrap

Text File | 1990-10-14 | 18.9 KB | 413 lines

********************************************************************* * NOTE: The following ASCII text file (without graphics) * * is contained in a printed technical paper available * * from Broadcast Electronics Inc. Unfortunately, it * * was not possible to reproduce the graphics portions * * of this paper within this text file. If you find the * * information in this file of interest, you may request * * a complimentary, printed, copy including figures and * * graphics from: BROADCAST ELECTRONICS INC. * * P.O. BOX 3606 * * 4100 N. 24TH STREET * * QUINCY, IL. 62305-3606 * * ATTN: SALES DEPARTMENT * * PH 217-224-9600 * * FAX 217-224-9607 * * * * The contents of this technical paper are * * Copyrighted (c) 1986, by Broadcast Electronics Inc. * * All rights reserved. * ********************************************************************* A MICROPROCESSOR PERFORMANCE OPTIMIZER FOR ALL TAPE FORMULATIONS BY: James R. (Rick) Carpenter Manager of Audio Engineering Broadcast Electronics Inc. Quincy, Illinois The proliferation of broadcast tape formulations has required the broadcast engineer to either accept performance degrading compromise set- tings for bias and equalization, restrict his use to only one tape formu- lation or equalize independently and frequently for each tape type. Even if the engineer chooses one tape type from one manufacturer, batch to batch variations in performance are inevitable. This paper describes a system that optimizes performance of various tape formulations rapidly and automatically. The system stores and re- trieves the optimum settings for a variety of tape formulations permit- ting maximum audio tape performance without time consuming manual adjust- ments for each and every recorded tape. BIAS, SENSITIVITY, AND EQUALIZATION VARIANCE IN BROADCAST CARTRIDGE TAPE Table 1 shows the statistical results of bias, equalization and sen- sitivity testing performed on three different groups of broadcast car- tridges on a single tape cartridge machine. The first group was new car- tridges of the same length from the same date code and the same manufac- turer. The second group was from the same manufacturer, but of different lengths, ages and date codes. The third group was a random selection of new cartridges from different manufacturers. TABLE 1. VARIANCE IN BROADCAST CARTRIDGE TAPE BIAS EQUALIZATION SENSITIVITY MEAN 3 MEAN 3 MEAN 3 GROUP1 61.7 0.32 dB 42 0.65 dB 35.4 0.6 dB GROUP2 62.9 0.96 dB 38.6 2.25 dB 34.9 1.15 dB GROUP3 59.1 2.30 dB 45.3 3.30 dB 34.5 2.40 dB As shown in Table 1, 99% of the first group of tapes were within 0.325 dB of the average bias current level of the group. 99% of the second group of tapes covered a range of 0.96 dB of the average bias current level. 99% of the third group of tapes were within 2.3 dB of the average bias level. 99% of the first group of cartridges were within 0.65 dB of the average equalizer level of the group. 99% of the second group of tapes covered a range of 2.25 dB of the average equalizer level of the group. 99% of the third group were within 3.3 dB of the average equalizer level of the group. The equalizer level is the amount of high frequency (12 kHz) gain necessary to match the 1kHz output level. 99% of the first group were within 0.6 dB of the mean sensitivity of the group. 99% of the second group were within 1.15 dB of the mean sensitivity of the group. 99% of the third group of tapes were within 2.4 dB of the mean sensitivity of the group. To demonstrate the audio performance impact of the statistical re- sults, -10 dB record/replay frequency responses were plotted. The bias, equalization and sensitivity were optimized for a tape on the mean of each group. The frequency response of other tapes in the group was then plotted without altering any parameters. These plots are shown in Figures 1, 2, and 3. The first group of new tapes, as would be expected from the statistical data and common sense, was the only group that had consistent response. Plots of the other, non-optimum groups, show large performance variations. With the normal station cart mix, optimum tape machine performance requires frequent and time consuming adjustments for each batch and type of tape cartridges. Automatic record alignment, with easy storage and retrieval of settings for various tape formulations and batches, is a valuable tool for the broadcast engineer. AUTOMATIC TAPE PARAMETER ADJUSTMENTS-THE LEARN MODE The learn mode of operation in the PT90RPS optimizes three critical recording parameters for any tape formulation: bias, equalization, and sensitivity. Criteria For Bias Adjustment There are many proposed criteria for defining the optimum bias level for a particular tape formulation and tape machine combination. The five least controversial criteria for optimum bias level settings are: 1. The bias level at which the third harmonic distortion is minimum. 2. The bias level at which the recorded sensitivity of a reference frequency is maximum. 3. The bias level at which the low frequency MOL is maximum. 4. The bias level at which the IM distortion is minimum. 5. The bias level at which the AM modulation distortion is minimum. FIGURE 1. SAMPLE OF FREQUENCY RESPONSE VARIATIONS - GROUP 1 TAPES FIGURE 2. SAMPLE OF FREQUENCY RESPONSE VARIATION - GROUP 2 TAPES FIGURE 3. SAMPLE OF FREQUENCY RESPONSE VARIATIONS - GROUP 3 TAPES Although sensitivity and distortion requirements cannot be satisfied simultaneously at all frequencies, the distortion and sensitivity versus bias of broadcast tape formulations at 7.5 ips form fairly broad curves as shown in Figure 4. In particular, the first two criteria mentioned above are almost equivalent. The LEARN mode was developed using a mod- ified version of criteria 1 and 2 above. This criteria leans toward re- duced noise and distortion at the expense of more complex record equali- zation circuitry. FIGURE 4. TAPE PARAMETERS VERSUS BIAS CURRENT As seen from Figure 5, tape sensitivity versus bias level plots usually have only one, easy to detect maximum. However, other factors of tape machine design and construction can cause false maximum detection. The two biggest causes of false peak detection are the inter-head time delay and tape dropouts. The record and play head gaps of a broadcast cartridge machine are separated by a nominal center-to-center distance of 1.125 inches. At the normal 7.5 ips tape speed, this is equivalent to a time delay of 0.15 seconds. Suppression of the dropout error requires that the bias be stepped in small increments (0.05 dB) and the playback output be recti- fied and filtered before input to the microprocessor. To accommodate the 3.75 ips speed and the time delay in the detection filter, the bias level is incremented at 0.5 second intervals. FIGURE 5. AUDIO OUTPUT MAXIMUM VERSUS BIAS CURRENT Bias Adjustments The bias adjustment is performed using the same audio detection and A/D circuitry used to establish the equalization and sensitivity levels. The internal 12 kHz oscillator is selected and the microprocessor incre- ments the bias D/A converters output in 0.05 dB steps. In order to re- duce the number of steps and therefore the amount of time needed to make the adjustment, the initial bias level is set to a predetermined, non- zero level. The bias current is increased until the playback audio level peaks. The peak audio level is multiplied by 0.86 (2 dB) and stored. The bias is then decremented until the playback audio level matches the stored -2 dB level. This is called "2 dB overbiasing". The frequency response difference of each tape is then compensated for with an adjust- able record equalizer. Figure 6 shows a flow chart of the bias adjust- ment system. FIGURE 6. LEARN MODE FLOW CHART BIAS ADJUSTMENT Tape Sensitivity Adjustment The tape sensitivity adjustment is performed using the same audio detection and A/D circuitry used to establish the bias level. The inter- nal 1kHz oscillator is selected and the audio gain VCAs in the input audio chain are incremented in 0.1 dB steps until a predetermined, user- defined level is reached. In order to reduce the number of steps and therefore the amount of time needed to make the adjustment, the initial level is set to a predetermined, non-zero level. The input sensitivity control has a range of 10 dB. The input sensitivity circuitry is fac- tory set to adjust for both the 160 nWb/M level or the 250 nWb/M level. The status of the deck elevated level sensor determines which level is used as the reference. Figure 7 shows a flow chart of the input sensitiv- ity adjustment system. FIGURE 7. FLOWCHART LEARN MODE SENSITIVITY ADJUSTMENT Record Equalization Adjustment The record equalization adjustment is performed using the audio de- tection and A/D circuitry used to set the bias and input sensitivity. The internal 12 kHz oscillator is selected and the VCAs in the input high frequency circuitry are incremented in 0.05 dB steps until the same pre- determined, user defined level used for the sensitivity adjustment is reached. In order to reduce the number of steps and therefore the amount of time needed to make the adjustment, the initial equalization level is set to a predetermined, non-zero level. The high frequency record equal- ization control has an adjustment range of 6 dB. Figure 8 shows a flow chart of the high frequency equalization adjustment. FIGURE 8. FLOWCHART LEARN MODE EQUALIZATION ADJUSTMENT The low frequency response of tapes is dominated by the response of the playback head. The low frequency "contour" effect causes response irregularities of as much as 1 dB in a well designed head and as much as 6 dB in older designs. Since the low frequency equalization adjustment is not as sensitive to tape formulation, it is controlled by a potenti- ometer adjustment with enough range to equalize the low frequency re- sponse for the most commonly used international equalization standards. The LEARN mode is designed to permit storage in battery-backed mem- ory of as many as 10 sets of bias, equalization and input sensitivity data for instant recall. The elevated level function permits automatic switching between two sets of data. For example, record settings can be automatically switched between a normal bias tape and elevated bias level tape. If the tape being optimized exceeds the range of the bias, equal- ization or level adjustments, the machine exits the LEARN mode, stops and the front panel displays the word "FAIL". A simplified flow chart for the entire LEARN mode is presented in Figure 9. The block diagram for the LEARN functions of the cartridge machine is given in Figure 10. The frequency response sweeps in Figure 11 show the performance of three different tape formulations after each was optimized using the LEARN mode. The sweep marked "A" is a normal bias tape. The sweep marked "B" is a high bias tape. The tape marked "C" is a new tape formulation requiring even higher bias than "B". The time required to optimize these tapes were: A=43 seconds; B=48 seconds; C=55 seconds. The time range to LEARN a tape was 42 seconds to 57 seconds for the tapes tested. FIGURE 9. SIMPLIFIED LEARN MODE FLOW CHART FIGURE 10. SIMPLIFIED BLOCK DIAGRAM OF LEARN MODE FIGURE 11. POST LEARN, PERFORMANCE OF THREE DIFFERENT TAPE FORMULATIONS MACHINE FEATURES The inclusion of a microcomputer in the cartridge machine has been the means of cost effectively adding other useful features. The PT90RPS includes a prioritized three level access to features, a real time tape timer, a manual adjustment mode, a built in tone oscillator (with sweep mode) a four digit alphanumeric display, a digital FSK tone encoder and the already mentioned LEARN mode. Three Level Function Access To insure the data integrity of the 10 memory registers, access to some functions of the cartridge machine can be restricted. There are three levels of access which can be programmed on the CPU module: 1. Normal Timer, recall memorized tape settings and produce recordings. 2. Learn Timer, settings can be recalled from memory and the LEARN (LRN) mode can be initiated to learn and save new tape settings to memory. 3. Manual All functions and settings can be accessed. Table 2. gives a summary of the three levels of function access. The characters in parenthesizes are the defaults for the front panel alphanumeric display. TABLE 2. LEARN MODE ACCESS LEVELS NORMAL LEVEL LEARN LEVEL MANUAL LEVEL Timer (TOOO) Timer (T000) Timer (T000) Recall Memory (RM#n) Recall Memory (RM#n) Recall Memory (RM#n) Learn (LRNn) Learn (LRNn) Fader (FLnn) (FRnn) Bias (BLnn) (BRnn) Equal (ELnn) (ERnn) Save Memory (SM#n) Oscillator ( OFF) Alignment (nnnn) Timer The four digit front panel timer is active in all modes when tape is running. It displays up to 59:59 minutes, but for the first 9:59 dis- plays a "T" in the leftmost digit. The timer will freeze at the end of the EOM displaying the message time and will display the total length of the cart by depressing the START switch. The timer can be programmed to accumulate time for multiple cuts. Manual Adjustment In order to permit individual tailoring of bias, equalization and level settings, the PT90RPS permits access to these adjustments when the CPU module is in the "MANUAL" setting. Adjustment to Bias Left (BL), Bias Right (BR), Fader Left (FL), Fader Right (FR), Equalization Left (EL) and Equalization Right (EL) is accomplished by pushing the Function (Func) and Execute (Exec) switches on the front panel. A numeric reading from 0-99 is displayed on the two rightmost digits (For example FR50). As the UP or Down (DN) switches are pressed, the numeric reading incre- ments or decrements. Once the adjustments are made they are then saved using the Save Memory (SM) and Execute switches. Once stored these set- tings can be used from any CPU level by using the Recall Memory (RM) function and the Execute switch. Tone Oscillator The PT90RPS divides the master microprocessor crystal to provide a very stable reference for cue tones and for a built-in tone oscillator. The oscillator gives front panel control of eight test tones (50 Hz, 125 Hz, 500 Hz, 1 kHz, 4 kHz, 8 kHz, 12 kHz and 16 kHz) with a frequency response of 0.25 db and distortion of less than 1%. Level of the tones is controlled by the front panel fader level controls. Depressing the UP or DOWN front panel switch for more than 3 seconds in test oscillator mode will initiate a sweep through the remaining test frequencies in that respective direction. CONCLUSION A microprocessor controlled broadcast tape cartridge performance optimization system has been profiled in this paper. This cost effective system finds, stores and retrieves the optimum settings for a wide vari- ety of tape formulations, permitting the engineer to optimize the sta- tions' audio performance, without time consuming manual adjustment for each and every tape. ACKNOWLEDGEMENTS The Author would like the thank T. Whiston for the tape testing, T. Lashbrook for the drawings and C. Steffen and L. Foster for putting this paper into readable form. THE AUTHOR James R. "Rick" Carpenter earned his BSEE, and is pursuing an MSEE, from West Virginia University in Morgantown, West Virginia. Mr. Carpenter has designed instrumentation for the U. S. Bureau of Mines. He was project engineer for the Harris MX-15 FM exciter develop- ment, the Broadcast Electronics TZ-30 TV MTS generator, and the Broadcast Electronics PT90 cartridge machine. The author has extensive design experience in solid-state RF design and analog equipment design. Mr. Carpenter has authored numerous technical papers, including co- authorship of the NAB Handbook chapter on "Analog Magnetic Recording" and is a member of the AES. The author is currently Manager of Audio Engineering for Broadcast Electronics Inc. in Quincy, Illinois. REFERENCES 1. Burstein, Herman, "How Important is Tape Azimuth", Audio VOL.68, No.9, pp. 40-746, 1984. 2. Kitmura, M., "A Method for Level Variance Analysis of Magnetic Tapes", AES Preprint 1816, 70th Convention, 1981. 3. Bealor, T., Carpenter, R., and Rosback, T., NAB Engineering Handbook Seventh Edition, (USA: National Association of Broadcasters, 1985), p. 5.11-217 - 5.11-237. 4. Budelman, G.A., "High-Frequency Variance: A Program-Dependent Deterioration Mode in Analog Magnetic Tape Recording", AES Preprint 1377, 60th Convention, 1978.