HW04

Design Constraint Analysis

Nick Schnettler

Team 7

ECE 477

Prof. Meyer

INTRODUCTION

This semester, Team 7 will be producing an MP3 Turntable. Using a compact flash card, the user will load files into the turntable system one at a time. Once loaded, the system will allow the user to manipulate the playback of an MP3 as if it were a vinyl record spinning on a turntable. Functionality will include tempo control, pitch control, looping, cueing, as well as standard playback features like play, pause, and stop.

The design constraints in this project are logically divisible into three classes: data transfer, user interface, and packaging. Data transfer constraints surface as a result of compact flash usage. User interface constraints are generated by ergonometric considerations, the need for sufficient tactile feedback, and the gathering of real-time data. Since the ultimate goal is to reproduce the look and feel of a vinyl turntable, packaging constraints arise because the entirety of the system must fit inside a portable case. Finally, as with most design projects, the primary constraint above all others is project cost: the project most operate within a collegiate student’s budget.

CONSTRAINT ANALYSIS

The MP3s will be loaded into the system from a compact flash card, an interface that does not support data streaming. Furthermore, manipulation of the file in play requires that the whole song be readily available to prevent gaps in playback. These restrictions introduce the first design constraint: the need for large microprocessor memory banks. The memory capacity of the chosen microprocessor must be roughly 5 to 6 megabytes to accommodate loading of an entire song. Given this much storage, compact flash data transfer restrictions are accommodated as are the restrictions imposed by the need for seamless playback.

The user operating the turntable will most likely be standing over the device. The line of sight from the user’s eyes to the system display will therefore be, on average, at least 3 feet. This introduces the second design constraint: the need for a system display large enough to be readable at a distance of 3 to 4 feet.

The system display is not the only portion of the user interface that generates design constraints. Though many of the features can be implemented with pushbuttons, (play, pause, stop, cue, and loop) the pitch/jog control must give the user a legitimate sense of moving a record back and forth. This necessity fosters two design constraints:the need to provide adequate tactile feedback to the user and theneed to gather rotational data as a velocity vector.

The packaging design constraint goes somewhat hand-in-hand with the user interface constraint of adequate tactile feedback. Not only does the user interface need to be comparable to that of a standard vinyl turntable, but it also needs to be compact enough to be portable like all other DJ equipment.
RATIONALE FOR COMPONENT SELECTION

Microprocessor: As was previously stated, a design requirement for the microprocessor was that have enough memory to accommodate loading of an entire MP3. According to its data sheets, the Rabbit 3000 microprocessor core module comes with 512K of SRAM and can accommodate 6 additional off chip memory modules. While this initially sounded adequate, it was later discovered that integration and addressing of these external memory modules would be an extremely arduous task because the Rabbit is only an 8-bit processor. The Ultimodule SCM220, on the other hand, offers 8MB of SRAM as well as 1MB of parallel flash 16MB of serial flash. Both processors have an adequate number of input pins for the peripherals that the project requires, butthe Ultimodule SCM220 was selected due to its vast memory banks.

Rotational Sensors: Original proposals for this project called for the use of an array of phototransistors (Panasonic SSG PNA1401L) in conjunction with a focused LED beam to sense rotational motion. The beam would be mounted on the underside of the turntable platter while the transistors would be installed along the inner wall of the platter bay. As the platter would rotate, the beam would pass over each transistor, and then the transistor data would be in turn fed to the processor for interpretation. While this scheme made sense on paper, the reality of project timing constraints (one semester for production) and budgetary constraints (phototransistors are roughly $4 a piece, making an array of them upwards of $40, not counting how many might be damaged in prototyping) motivated a search for cheaper solutions. The resulting find was a Grayhill Rotary Pulse Generator (series 25L). Mounted at the base of the turntable platter, this inexpensive device will provide 36 discrete of data points per rotation in addition to directional indications. Selecting the RPG over the phototransistors simplified the gathering of rotational data while still allowing for easy interfacing to the tactile feedback mechanism desired.

System Display: Before considering line-of-sight/ergonometric issues, the display selected was the Crystalfontz CFAG12864B-TMI-V 128x64. The original thought was that a backlit display such as this would take care of any line-of-sight issues. The more it was discussed, however, the more it became apparent that backlighting wasn’t enough – if the font wasn’t big enough, the user would have to crouch over the display to read it. Judging that this was unacceptable, we chose the PJRC 24x8 MP3 player display. Though this model isn’t backlit (the backlit version of this model was over $100), the size of the font is considerably larger than that of the Crystalfontz model. To keep cost down, the number of characters that can be displayed is lower than that of the Crystalfontz, but it was decided that this was an acceptable tradeoff. An added benefit is that the PJRC model comes with a 12 key pushbutton board, a feature which simplifies our user interface significantly.

MP3 Decoder/Digital to Audio Converter: While these components do not contribute directly to the design constraints, they are an integral part of the design, and therefore merit mention. Initial plans called for use of the Micronas MAS 3507D MPEG Decoder in conjunction with the Micronas DAC 3550A Stereo DAC. These components were sold as a package from Micronas, and this was attractive from the standpoint that interfacing notes came with the package. It was later discovered, however, that the Micronas line of MPEG decoders was unreliable. We replaced that pairing with the ST STA013 MPEG 2.5 Decoder and the Cirrus Logic CS4334 Stereo D/A Converter. The ST MPEG Decoder is noted as interfacing well with the Cirrus D/A Converter, which was again attractive. In addition, the ST MPEG Decoder features a variable speed internal clock, a feature which should make tempo variations fairly simple to implement.

LIST OF MAJOR COMPONENTS

Vendor / Part# / Description / Price / Quantity
Ultimodule / SCM220 / IDT MIPS CPU
Starter Kit / FREE / 1
PJRC.com / MP3_DISPLAY / LCD Display
24x8 with
pushbuttons / US$42 / 1
PJRC.com / STA013_SOP28 / STA013 MP3
Decoder / US$12 / 1
NewarkInOne.com / 25LB10-Q / Grayhill
36 position
Mechanical
Encoder / US$4 / 1
NewarkInOne.com / CS4334-BS / Cirrus Logic
24 bit Stereo
D/A Converter / US$3 / 1

LIST OF REFERENCES

Ultimode SCM220 Starter Kit – [AVAILABLE]

Rabbit 3000 Microprocessor–

[AVAILABLE]

LCD Display by PJRC.com -

[AVAILABLE]

Crystalfontz CFAG12864B-TMI-V –

[AVAILABLE]

ST STA013 MPEG Decoder –

[AVAILABLE]

Micronas MAS 3507D MPEG Decoder –

[AVAILABLE]

Cirrus Logic CS4334 D/A Converter –

[AVAILABLE]

Micronas DAC 3550A –

[AVAILABLE]

Grayhill Series 25L Mechanical Encoder –

[AVAILABLE]

Panasonic SSG PNA1401L Phototransistor –

[AVAILABLE]